Chapter 7: Immunopathology

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You know, we spend so much time praising the human immune system.

And I mean, for good reason.

It's this incredibly sophisticated, billion -year -old fortress that protects us from a world that is essentially trying to eat us.

It really is.

It's the ultimate biological super weapon.

It's intelligent, it learns, and it has a memory.

It's amazing.

Right.

But there's a darker side to owning a super weapon, isn't there?

If the targeting system glitches or if the trigger gets stuck, you don't just get a minor error.

You get

catastrophe.

That is the perfect way to frame it.

Today isn't about the immune system working properly.

It's all about when it fails.

We're diving into immunopathology, which is chapter seven of the lecture notes.

And honestly, this is one of the most dramatic narratives in all of medicine.

It really is.

I was reading through this chapter and it reads like a war novel.

You've got friendly fire, you have infiltrators, you have total structural collapse.

It's intense.

And the stakes are life and death.

We're not just talking about a runny nose here.

We're talking about the body liquefying its own kidneys or, you know, lungs shutting down in seconds because of a peanut.

Or the complete erasure of the body's ability to defend itself at all.

Exactly.

So here's the battle plan for this deep dive.

The source material breaks us down into five distinct theaters of war, let's call them.

We have hypersensitivity, then autoimmunity, primary deficiency,

AIDS, and finally transplant rejection.

That's a massive amount of ground to cover.

It is massive.

So we're going to take our time and really pull apart the mechanics.

We want to get past the memorization and get to the why.

Why does the body do this?

How does a single molecule trigger a chain reaction that, you know, shuts down an airway?

That's the mission, to really understand the story behind the pathology.

Let's start with part one, hypersensitivity.

Now, in common language, being hypersensitive just means you get your feelings hurt easily.

But biologically, this is a very specific type of violence.

It is.

In pathology, hypersensitivity is essentially an exaggerated, inappropriate immune response that ends up injuring the host.

It's the immune system using a bazooka to kill a fly.

And taking out the living room wall in the process.

That's it, exactly.

Okay.

The text categorizes these into four types.

And I want to spend some real time on type I because this is the one everyone thinks they know, but the actual mechanism is, well, it's fascinatingly dangerous.

This is immediate or anaphylactic hypersensitivity.

The classic allergy, bee stings, peanut allergies, that sort of thing.

Right.

But let's look at the cellular level.

The text mentions this reaction is IgE dependent.

Help me understand the context here.

We have different types of antibodies, IgG, IgM, IgA.

Why is IgE the villain in this story?

Well, traditionally, IgE wasn't a villain at all.

Evolutionarily, IgE was our main defense against parasites, helminths, worms,

you know, big invaders.

If you have a tapeworm, you need a massive explosive response to try and flush it out of your system.

Okay.

So IgE is designed for shock and awe warfare.

Precisely.

But in the modern sanitized world, we just don't have as many tapeworms.

IgE is basically this unemployed, highly trained super soldier who's just looking for a fight.

And sometimes it picks a fight with something completely harmless like pollen or cat dander.

That is a terrifying thought.

So walk me through the sequence.

Let's say I eat a peanut.

I have the allergy.

What happens in the first few seconds?

Okay.

So visualize this.

Deep in your tissues, lining your blood vessels and your airways, you have these sentry cells.

They're called mask cells and basophils.

I always picture these as like floating naval mines, just waiting.

That's not far off at all.

They are packed, literally stuffed with granules that contain these incredibly potent chemicals.

Now, normally these cells are just waiting.

But if you're allergic, your mass cells are already primed.

They are totally coated in IgE antibodies that are specific to that peanut protein.

So they're wearing a custom -made coat designed just to catch peanut protein.

Exactly.

The antibody is bound to the cell through something called an FC receptor.

It's locked and loaded.

The moment that peanut antigen enters your bloodstream and touches those antibodies,

it causes them to cross -link.

Cross -link?

What does that mean?

You imagine holding hands.

The antigen is big enough to bridge two antibody arms together.

It physically connects them.

That physical connection sends a shockwave signal into the mass cell saying detonate.

And that's the degranulation the book talks about.

Right.

And degranulation is such a polite clinical word for what's happening.

It's essentially a chemical explosion.

The cell membrane rips open and just dumps this cocktail of misery into your surrounding tissue.

Let's look at that cocktail list because the notes break this down into preformed and newly synthesized.

The preformed stuff is basically sitting in the chamber ready to fire.

Yes.

The moment that trigger is pulled, boom, histamine is out.

Also, heparin.

And histamine is the one responsible for the immediate swelling and itching, right?

Histamine is a powerful vasodilator.

It blasts the blood vessels open.

That makes them super leaky.

Fluid rushes out into the tissue.

That's your swelling, your edema.

It also stimulates nerve endings, which is why you get that intense unbearable itching.

But the text says histamine isn't the only preformed thing in the chamber.

There's also something called eosinophil chemotactic factor.

This is absolutely crucial for understanding why allergic reactions can linger.

This is the distress flare.

The mast cell isn't just attacking.

It's screaming for backup.

It's sending out a signal to eosinophils to come to the battlefield.

But they're not there instantly.

They take a while to arrive.

Right.

Which explains why these reactions have phases.

Exactly.

You have the immediate phase.

That's the histamine hitting you right now.

Phasodilation, bronchospasm, swelling.

But then, hours later, those eosinophils that got the signal finally arrived.

That's the late phase.

That's the late phase.

And eosinophils are nasty customers.

They release enzymes that cause a lot of tissue damage and they keep the inflammation going for hours.

This is why an asthma attack can seem to get better and then come roaring back hours after the initial trigger.

Okay, that makes sense.

I want to look at the newly synthesized list in the notes.

These are the chemicals the mast cell cooks up after it gets triggered.

Leucotrienes and prostaglandins.

These are the heavy hitters.

Specifically, the text points out

leucotrienes C4 and D4 and prostaglandin D4.

It sounds like explosive ordinance.

C4.

It pretty much acts like it.

In the lungs, histamine initiates the constriction.

But leucotrienes are thousands of times more potent at causing bronchospasm than histamine is.

They clamp the airways shut.

Hard.

So if I'm having an anaphylactic reaction, the histamine is what's making me swollen and itchy, but the leucotrienes are the reason I literally can't breathe.

Effectively, yes.

They are the primary driver of that life -threatening bronchoconstriction.

And that's why epinephrine is the treatment.

It forcefully reopens those airways and clamps down the blood vessels to stop the leak.

It directly reverses the mechanics of those mast cell mediators.

Wow.

Okay, so that's type I.

Let's move on to type II.

This is antibody -mediated.

How is this distinct from what we just talked about?

It's all about the target.

In type I, the antibody, the IgE, is sitting on a mast cell waiting for a floating antigen to come to it.

In type II, the antibodies, usually IgU or IgM, are on the hunt.

They are targeting antigens that are fixed on a cell surface.

Fixed is the key word there, then.

Right.

The target isn't floating pollen.

The target is a cell in your own body.

Maybe it's a red blood cell.

Maybe it's a thyroid cell.

The antibody finds it, binds to it, and basically paints a target on its back.

The notes list three ways this leads to destruction.

The first one is complement dependent.

Okay.

Think of the complement system as a biological drilling team.

When the antibody binds to the cell, it calls in these complement proteins.

They assemble on the cell surface and literally punch a hole in the membrane.

That's the membrane attack complex, or MAC.

MAC.

It creates a hole.

Water rushes in, osmotic lysis, and the cell just pops.

It explodes.

Or the complement system can just obscenize the cell.

Which is a fancy word for making it pasty for other immune cells.

It's putting a little flag on the cell that says, eat me, to the nearest macropage.

Yeah.

This is exactly what happens in transfusion reactions.

You give someone the wrong blood type, their antibodies bind to the new red blood cells, complement activates, and those cells are either popped or eaten.

The text also lists erythroblastosis fatalis here.

It's the same mechanism.

Maternal IgG antibodies cross the placenta and attack the fetus's red blood cells because of an Rh incompatibility.

The baby's cells are obscenized and destroyed by its own mother's immune system.

That's tragic.

The second mechanism listed is ADCC, antibody dependent cell mediated cytotoxicity.

That is a mouthful.

It is, but the concept is pretty simple.

The antibody coats the target cell.

But instead of complement coming in, other immune cells like natural killer cells or macrophages dock onto the antibody and inject toxins directly into the target cell.

It's an execution.

It's a direct, up -close, and personal execution.

The classic example given in the notes is pernicious anemia, where the body makes antibodies that target the parietal cells in the stomach lining.

Now the third mechanism in type 2 is, it's a bit weird, because it doesn't necessarily kill the cell, right?

Anti receptor antibodies.

Right.

This is where the immune system starts hacking the body's communication lines.

The antibody binds to a receptor on a cell surface.

But instead of destroying the cell, it just messes with the signal.

It either blocks it or, even weirder, it mimics it.

And the classic example here is Graves' disease.

Graves' is fascinating.

You have an antibody that binds to the TSH receptor on the thyroid gland.

But instead of blocking it, it activates it.

It basically pretends to be the hormone signal from the brain.

So it's creating a false flag signal telling the thyroid to just produce hormone constantly?

Constantly.

The thyroid thinks the brain is screaming, more hormone, more hormone.

But it's just this rogue antibody hot -wiring the system so the patient becomes severely hyperthyroid.

And on the flip side of that, you have myasthenia gravis.

Exactly the opposite effect.

The antibody binds to the acetylcholine receptor on the muscle.

But this time, it blocks the signal.

The nerve tries to fire, it sends the signal, but the muscle never gets the memo.

So you get progressive, profound weakness and paralysis.

So type 2 is the sniper.

One specific target, one direct shot.

That's a perfect way to put it.

Which brings us to type 3.

Immune complex disease.

If type 2 is the sniper, type 3 feels more like shrapnel.

Indiscriminate damage.

That's a great analogy.

Type 3 is, at its heart, a plumbing problem.

A plumbing problem.

Yeah, here's the setup.

You have soluble antigen and soluble antibody meeting in the blood.

They lock together to form what's called an immune complex.

Normally, the spleen or the liver would clear these little complexes out of circulation.

But in type 3...

For whatever reason, there are too many of them, or they're just the wrong size, they'll get cleared properly, so they just keep circulating.

And eventually, like sediment in a pipe, they get stuck.

And where do they tend to get stuck?

In the body's natural filtration systems.

So the kidneys causing glomerulonephritis, the joints causing arthritis, or just the walls of small blood vessels causing vasculitis.

Figure 701 in the notes really visualizes this.

I see the complex just wedge into the vessel wall.

And that's just the start of the problem.

Once it's wedged there, it activates complement.

But here's the issue.

The complex is stuck to your own tissue.

So when the neutrophils rush in to attack the complex, they end up releasing all their lysosomal enzymes right onto your own vessel wall.

So the vessel wall is just collateral damage in this fight.

It's the definition of collateral damage.

It causes something called fibroinoid necrosis.

The immune system is trying to clean up a mess, but it ends up destroying the filter itself.

This is the core mechanism behind systemic lupus erythematosus, or SLE,

and serum sickness.

Okay, we have one more hypersensitivity type, type 4.

And this one is the odd one out.

It really is.

Types I, II, and III are all mediated by antibodies, which means they're a B -cell game.

Type V is cell -mediated.

It's a T -cell show.

And because it involves recruiting T -cells, it's not immediate, it's slow.

Right.

That's why it's also called delayed type hypersensitivity.

Think about the TB skin test, the PPD test.

You get the injection, but the doctor doesn't look at it right away.

They tell you to come back in 48 to 72 hours.

Because it takes that long for the T -cells to actually travel to the site and mount a response.

Exactly.

CD4 -positive helper T -cells have to recognize the antigen, get activated, migrate to the skin, and then start releasing cytokines to call in the macrophages.

It's a slow mobilization of the infantry.

And what does it look like when they finally get there?

You get a granuloma formation.

The macrophages surround the emptor and wall it off.

It's a hard red raised bump.

That's the positive TB test you see.

The notes also mention a cytotoxic form of type IV.

Yes.

That involves the other kind of T -cell, the CD8 -positive killer T -cells.

This is what happens in type I diabetes.

The T -cells mistake the beta cells in the pancreas for enemies,

and they systematically hunt them down and execute them.

It's frighteningly efficient.

It is.

So just to recap the big four in simple terms.

TICON is the grenade mass cells in IgE.

Type II is the sniper antibody versus a specific cell.

Type III is the shrapnel complexes getting stuck.

And type IV is the infantry invasion T -cells arriving late to the party.

That framework makes it so much easier to visualize.

And that leads us perfectly into the next section.

Because if hypersensitivity is the system overreacting to a specific trigger, this next category is the system completely losing its ability to distinguish friend from foe.

Section II, autoimmune diseases,

the body versus itself.

And we have to start with the prototype, the one they call the great imitator, systemic lupus erythematosus.

SLE.

Yeah, this is a disease that really highlights the tragedy of autoimmunity.

It is a multi -system catastrophe caused by a total loss of what we call self -tolerance.

The demographics here are incredibly stark.

The notes say the female to male ratio is 9 to 1.

Think about that.

90 % of patients are women.

And specifically, women of childbearing age, usually between 20 and 45.

Is there a hormonal link then?

There almost certainly is, given the timing and that extreme gender disparity.

Though the exact mechanism is still really complex.

African Americans are also at a higher risk.

But clinically, if you see a young woman presenting with vague multi -system complaints, you know, joint pain, a rash, fatigue lupus has to be high on your radar.

SLE.

Let's talk about the mechanism.

It's actually a hybrid of the hypersensitivities we just talked about, isn't it?

It is.

It's primarily a combination of type II and type III.

You have type II antibodies attacking blood cells that causes anemia and low platelets.

And you have those type III immune complexes getting stuck everywhere, especially in the kidneys and the skin.

The diagnosis seems to rely so heavily on this alphabet soup of antibodies.

I want to parse these out because on a board exam or in the clinic, knowing the difference is just critical.

Absolutely.

The first gatekeeper test is the ANA, the anti -nuclear antibody.

And this is the sensitive one, right?

Extremely sensitive.

The tech says greater than 95 % of lupus patients have a positive ANA.

So if the ANA is negative, you can be pretty confident it's not lupus.

It's a fantastic screening tool.

But is it specific?

Not at all.

You can have a positive ANA from a dozen other autoimmune diseases, from viral infections, or even just from aging.

So a positive ANA tells you something autoimmune is happening, but it doesn't tell you it's definitely lupus.

For that, we need the more specific markers.

Right.

And those are anti -DS DNA, which is anti -double -scranded DNA, and anti -TILVM, the Smith antigen.

If you see these, you basically have your diagnosis.

They're highly specific for SLE.

There's a clinical correlate box in the notes about drug -induced lupus.

Yes.

This is a fascinating phenomenon where certain medications can actually trigger a lupus -like syndrome.

Which drugs are we talking about?

The classic triad to memorize is hydrolazine, which is an old blood pressure medication,

prokainamide for cardiac arrhythmias, and isoniazid, which is a key drug for treating tuberculosis.

And there's a specific antibody for this version of it?

Yes.

Anti -histone antibodies.

If you have a patient on, say, hydrolazine, who develops a rash and joint pain, you check the anti -histone antibody.

If it's positive, you stop the drug, and the lupus almost always goes away.

That's a crucial catch.

Now lupus attacks everything.

I mean, systemic is right there in the name.

Let's walk through the systems defined in the text because the presentation can be so incredibly varied.

Let's start with hematologic.

That's the type 2 reaction we mentioned.

The body literally eats its own red cells, causing anemia, its own platelets, causing thrombocytopenia, or its white cells.

Okay, skeletal.

You get arthritis.

But here is the key distinction from rheumatoid arthritis.

In lupus, the arthritis is non -deforming.

The joints hurt.

They're inflamed.

That's synovitis.

But they don't get destroyed and twisted the way they do in RA.

And the skin.

This is the famous image everyone knows.

The malar rash.

The butterfly rash.

It covers the cheeks and the bridge of the nose.

And interestingly,

it characteristically spares the nasolabial folds, the left lines right next to your nose.

That's a classic board exam visual.

And finally, the really dangerous ones, the heart and kidneys.

In the heart, you can get something called Libman -Sachs endocarditis.

The text describes these as non -bacterial varicose endocarditis.

They're like little sterile warts that form on the heart bells.

But the kidney.

The kidney is the real danger zone.

It is.

Lupus nephritis is the major cause of morbidity and mortality in these patients.

The text actually asks us to know the classification system for it.

Class I through six.

Why do we need to know all the different classes?

Because the treatment and the prognosis change drastically depending on the class.

Class I and II are pretty mild.

But class III, which is focal, and especially class IV, which is diffuse, are very aggressive.

Break down class IV for me.

What does that mean?

Class IV is diffuse proliferative lupus nephritis.

Diffuse means it involves more than 50 % of the glomerulate, the little filtering units in the kidney.

This is both the most common and the most severe form.

These patients need aggressive immunosuppression like chemotherapy to save their kidneys.

And class V.

That's membranous.

That involves a thickening of the basement membrane of the glomerulus.

So what's class VI?

Class VI is advanced chlorosing.

That's basically game over for the kidney.

It's all scarred down and useless.

The entire goal of treatment is to stop the patient from ever reaching class VI.

It's just such a heavy diagnosis.

The text mentions a 10 -year survival of about 85%, which is better than it used to be.

But death still often comes from either renal failure or infections caused by the powerful immunosuppressive treatments themselves.

It's a very difficult balance to strike.

Let's look at the other autoimmune conditions in this section.

Next up is Sugarin syndrome.

I always remember this one as the dry syndrome.

The sicka syndrome, yeah.

The immune system targets two very specific things.

The lacrimal glands, which make tears, and the salivary glands, which make spit.

It sounds like a nuisance on the surface, but it's actually quite damaging.

It is very much so.

Dry eyes carotid conjunctivitis sicka can lead to painful corneal ulcers because you don't have that protective tear film.

And dry mouth xerostomia leads to rampant tooth decay and difficulty swallowing.

The text mentions Maculet syndrome.

What's that?

That's the physical enlargement of the glands.

You can actually see the swollen parotid glands in the cheeks, making them look puffy.

What are the key antibodies we're looking for here?

The two big ones are anti -SSA, also called Rho, and anti -SSB, also called La.

These are anti -ribonucleoprotein antibodies.

There's a really scary warning here for pregnant women with anti -Rho antibodies?

Yes.

This is a huge deal.

If a pregnant woman has anti -Rho, those antibodies can cross the placenta.

When they get to the fetus, they attack its developing heart conduction system.

This can cause congenital complete heart block.

The baby is born with a heart that can't conduct electricity properly.

It can't beat properly.

That is just devastating.

And there is one more risk for the patient themselves, right?

A big one.

B -cell lymphoma.

If a Sjogren patient has had stable swollen glands for years, and then suddenly one gland gets much, much bigger, you have to biopsy it immediately.

The risk of developing that specific type of cancer is significantly higher in these patients.

Okay.

Moving on to scleroderma, or systemic sclerosis.

This is a disease of hardening.

Sclero means hard, derma means skin.

But it's not just skin deep.

It's a systemic fibrosis problem.

What's the driver here?

What's causing the fibrosis?

It's fibroblast activation.

The immune system releases a flood of cytokines IL -1, PDGF, FGF.

These are all signals that basically tell fibroblasts to build, build, build.

So just lay down tons and tons of collagen everywhere.

And the skin, the lungs, the gut.

The text splits this into two major types.

Diffuse and localized.

This distinction seems absolutely vital for prognosis.

It is night and day.

Diffuse scleroderma is the really bad actor.

It has widespread skin involvement and crucially early visceral involvement.

Visceral meaning the internal organs.

Yes.

The lungs get fibrosed, you can't breathe.

The heart gets fibrosed, you get arrhythmias.

The kidneys fail.

The antibody marker to know here is anti -DNA to poismarase I, also called CL70.

And you contrast that with localized scleroderma, which has another name, Crest syndrome.

Right.

And this is a much more benign course.

The skin tightening is usually limited to the face and fingers.

And the antibody is different too.

It's anti -centromere.

Crest is a mnemonic.

I want to walk through because each letter paints such a clear picture of the patient.

C, calcinosis.

You get these little painful hard calcium deposits forming in the skin, especially on the fingertips.

Raynaud phenomena.

This is usually the very first sign.

In the cold, the blood vessels in the fingers spasm.

The fingers turn white from ischemia, then blue from hypoxia, then finally bright red when the blood rushes back in.

Esophageal dismotility.

The esophagus gets stiff from all the collagen.

Patients have a lot of trouble swallowing solid food.

It feels like it gets stuck.

Sclerodactyly.

This is the classic claw hand.

The skin on the fingers gets so tight and shiny that you literally can't bend the fingers properly.

They get stuck in a flex position.

And finally, T?

Telangiectasia.

These are little dilated blood vessels.

They look like small red spider marks that appear on the skin of the face or hands.

So if you see the claw hand and the red spots on the face, you should be thinking crest.

Absolutely.

Okay, finally, in this section, we have mixed connective tissue disease.

And this is exactly what the name implies.

It's a mix.

The patient has some features of lupus, some features of sclerosis, and some features of polymyositis, which is muscle inflammation.

It's an overlap syndrome.

And the tiebreaker,

the key lab test.

High titers of an antibody to U1 ribonucleoprotein.

That's the specific marker for this overlap disease.

All right, that wraps up the section on the body attacking itself.

Now let's flip the script entirely.

Section three, primary immune deficiency syndromes.

These are people who are essentially born vulnerable.

These are arguably even more tragic because they're genetic.

The blueprint for the immune system is just missing a critical page from birth.

Let's start with X -linked agammaglobulinemia of Bruton.

That is a very long name.

Let's break it down.

Agammaglobulinemia, no gammas, no immunoglobulins, so no antibodies.

X -linked, it almost exclusively affects boys.

What's the specific genetic defect?

It's a mutation in the BTK gene, which stands for Bruton tyrosine kinase.

This enzyme is an essential signal for B cells to grow up.

Without it, the pre -B cells that are made in the bone marrow can never become mature B cells.

So if you have no mature B cells?

You have no plasma cells.

Then if you have no plasma cells?

You have no antibodies.

Zero.

None.

The text notes the onset of symptoms is around six months of age.

Why not right at birth?

That's the maternal shield.

For the first six months of life, the baby is protected by circulating mom's IgG antibodies that cross the placenta during pregnancy.

But as those fade away, the baby's own system completely fails to take over.

And what kind of infections hit them first?

Recurrent bacterial infections.

Specifically, encapsulated bacteria like H influenza, streptococcus pneumonia,

staph aureus.

The bacteria that antibodies are supposed to tag for destruction just run wild.

How does that compare to common variable immunodeficiency or CVID?

CV is similar in that you have low antibodies,

specifically IgA and IgG.

But the mechanism is a defect in B cell maturation that's, well, it's more complex and we don't fully understand it yet.

It also presents later, usually in childhood or even early adulthood.

And the infection profile is a little different.

Yes.

Bacteria for sure.

But the text also points out a susceptibility to Giardialamblia.

That's a parasite that causes horrible diarrhea.

And paradoxically, these patients are at a much higher risk for developing autoimmune diseases and lymphoma.

Okay.

Next is deGeorge syndrome.

This one takes us all the way back to embryology class.

It does.

This is a failure of the third and fourth pharyngeal pouches to develop properly in the embryo.

What are those pouches supposed to become?

They're the construction site for two very important organs.

The thymus gland and the parathyroid glands.

So deGeorge patients are often born without them.

Let's look at the consequences of each.

No parathyroids means what?

The parathyroids control calcium levels.

Without them, you get severe hypocalcemia.

Low calcium causes tetany involuntary muscle spasms and contractions.

This is often how these babies present in the nursery with seizures.

And no thymus.

The thymus is the school where T cells go to mature and learn to tell self from non -self.

No school means no educated T cells.

So you have a profound T cell deficiency, which leaves them vulnerable to?

Viral and fungal infections.

Remember, B cells and their antibodies are great for bacteria floating around.

Key cells are the viral and fungal hunters.

Now we get to the absolute worst case scenario of these diseases.

SCID, severe combined immunodeficiency.

This is the bubble boy disease.

The combined in the name refers to the fact that both arms of the adaptive immune system, humeral, the B cells, and cellular,

the T cells are completely wiped out.

The text lists two main causes.

Let's distinguish them.

First, there's an X -linked version.

The X -linked version is a mutation in the cytokine receptor gamma chain.

This little chain is a critical component for the receptors for multiple interleukins.

IL -2, IL -4, IL -7.

These are the growth signals for all lymphocytes.

Without the receptor, the cells can't get the signal and they effectively starve and die before they can mature.

And the other cause is autosomal recessive.

This involves an enzyme called adenosine deminase or ADA.

Right, and this is more of a metabolic poison problem.

ADA is an enzyme that helps break down purines from DNA.

If you lack ADA, a substance called deoxyadenosine builds up inside your cells.

And deoxyadenosine is bad news for the immune system.

It is uniquely toxic to lymphoid progenitor cells.

It poisons the baby's immune system from the inside out before it can even develop.

What's the prognosis for SCID?

Without a bone marrow or stem cell transplant, these children die within the first year of life.

They have absolutely zero defense against the world.

Next on the list is Whiskott -Aldrich syndrome, another X -linked one.

This has a classic triad of symptoms.

If you see a baby boy with this combination, you have to think of it.

Number one, eczema.

Two, thrombocytopenia, which means low platelets causing easy bruising and bleeding.

And three, recurrent infections.

So eczema bleeding infections.

The classic triad.

It's caused by a mutation of the YSP gene.

Okay, let's do a quick run of the complement disorders to finish this section.

If you're missing the early protein C1Q, C2, C4, you get a lupus -like immune complex disease.

If you're missing the C1 esterase inhibitor, you get hereditary angioedema, which is that scary swelling of the face and airway without any hives.

And the late ones?

If you're missing C5 through C9, you can't form the MAC, the membrane attack complex.

This leaves you uniquely susceptible to recurrent Neisseria infections.

That's meningitis and gonorrhea.

And one last deficiency that's really common, selective IGA deficiency.

This is the most common primary immunodeficiency.

Most people are actually asymptomatic and don't even know they have it.

But since IGA protects mucosal surfaces, they might get more sinus infections or diarrhea.

But there's a huge warning here in the notes regarding blood transfusions for these patients.

Yes, this is critical.

Because they have never seen IGA in their entire lives, their body thinks IGA is a foreign invader.

If you give them a blood transfusion that contains someone else's IGA, they can mount a massive life -threatening anaphylactic reaction to it.

Wow, that is a landmine.

Okay, that brings us to section four, acquired immunodeficiency syndrome, AIDS.

This is a modern plague and it's caused by HIV, the human immunodeficiency virus, which is an enveloped RNA retrovirus.

Let's get the definition straight first.

HIV is the virus.

When does an infection officially become AIDS?

The text gives very specific criteria.

You are defined as having AIDS when you are HIV positive and your CD4 T cell count drops below 200 cells per microliter.

Or alternatively, if you're HIV positive and you develop any one of a list of specific AIDS -defining illnesses, regardless of your CD4 count.

I want to zoom in on the entry mechanism.

Figure 7 -3 depicts this kind of molecular handshake that lets the virus in.

It feels like a heist.

Movie like the virus is picking a very complex lock.

That's a great way to visualize it.

It's a two -step process.

Step one, the virus approaches a T cell.

It has a key on its surface called GP120.

It sticks this key into the first lock, which is the CD4 receptor on the host T cell.

But the door doesn't open yet, right?

No.

Turning that first key causes a conformational change.

The GP120 protein shifts its shape.

This reveals a second part of the key that can now fit into a second lock, a co -receptor.

And those co -receptors are CCR5 or CXCR4.

Exactly.

Once GP120 binds to that co -receptor, the door is fully unlocked.

Then, step two, a different viral protein, GP41, acts like a grappling hook.

It shoots out, harpoons the cell membrane, and physically pulls the virus in until the two membranes fuse and the viral contents are injected.

And once it's in, it begins the infection.

The timeline here has three distinct phases.

Phase one is the acute phase.

This is right after infection.

The amount of virus in the blood, the viremius spikes.

The patient feels like they have the flu or monofever, swollen lymph nodes, sore throat.

And then silence.

Phase two, the clinical latency period.

The patient feels completely fine, they're asymptomatic.

But inside, a massive war is raging in the lymph nodes.

The virus is replicating furiously, and the immune system is desperately trying to hold the line.

This can last for an average of 10 years.

But eventually, the line breaks.

That's phase three, progression to AIDS.

The CD4 count finally crashes below 200.

The immune system is exhausted and collapses.

The walls are down.

And the opportunists come storming in.

Table 7 -1 is this terrifying list of things that shouldn't kill you.

But in an AIDS patient, they absolutely do.

Let's group them.

What hits the lungs?

The classic one is Pneumocystis Girovichia pneumonia, or PJP.

It causes a suffocating pneumonia.

Also, reactivated TB and fungal infections like histoplasmosis.

The CNS, the brain.

You see things like toxoplasmosis, which causes ring enhancing abscesses in the brain.

Cryptococcus, which causes a devastating meningitis.

And the JC virus, which causes PML progressive multifocal leukoencephalopathy, where the white matter of the brain is literally destroyed.

The GI tract.

Candida, causing thrush.

It lines the mouth and esophagus with these thick white plaques.

And cryptosporidium, a parasite that causes a relentless chronic watery diarrhea.

And the skin has a very specific and visible marker.

Kaposi sarcoma.

Describe figure 7 -4 for us.

What does Kaposi sarcoma look like?

It shows these violaceous, which means purple or dark red patches, plaques or nodules on the skin.

This isn't just a rash, it's a neoplasm, a cancer of blood vessels.

It's caused by a co -infection with HHV8, human herpes virus 8.

And finally, lymphoma.

Yes, a much higher risk of high -grade B -cell lymphomas.

Especially primary CNS lymphoma, which is very rare in people who aren't immunocompromised.

The virus disrupts the immune surveillance that normally stocks these cancers from ever starting.

It's just a devastating systematic failure of the entire system.

It really is.

Okay, we are in the homestretch now.

Section 5.

The immunology of transplant rejection.

This is the only section where the invader is actually something that's trying to help us.

That's true.

We're putting a new life -saving organ in, but the immune system just sees it as foreign.

It recognizes the donor's HLA alleles as non -self and mounts an attack.

There are three types of rejection, and the key to understanding them is the timeline.

Let's start with the fastest one, hyperacute rejection.

This happens in minutes to hours.

The surgeon connects the vessels, unclamps them, blood flows into the new kidney, and right there on the operating table, before their eyes, the organ turns black and dies.

That is an absolute horror story.

Why does it happen so incredibly fast?

Because the weapons were already there.

The recipient had preformed antibodies in their blood against the donor's tissue.

Maybe from a previous pregnancy, a blood transfusion, or a previous transplant.

Those antibodies bind immediately, activate complement, cause widespread thrombosis, and kill the organ instantly.

This is why we do cross -matching before transplants now.

Exactly.

It's thankfully rare today because we check for those pre -existing antibodies first.

Okay, next up is acute rejection.

This is the most common type.

It happens within the first six months after transplant.

This is the immune system waking up and realizing, hey, wait a minute, this kidney doesn't belong here, and then mounting a fresh attack.

Is it two cells or antibodies leading the charge?

It can be both.

You can have a cellular rejection, which involves CD8 plus T cells directly infiltrating and killing the graft tissue.

Or you can have a humoral rejection, which involves the patient making new antibodies against the graft.

But the key is that it's often reversible if you catch it early and increase the immunosuppressants.

Finally, we have chronic rejection.

This is the real heartbreaker.

It happens slowly over months to years,

and you can't really stop it.

You can only try to slow it down.

It's the main reason transplants eventually fail.

What's the mechanism here?

What's happening?

It attacks the plumbing.

It's a slow smoldering reaction where T cells and antibodies react against the blood vessels of the graft itself.

This causes fibrosis.

The vessel walls get thick and scarred, and the lumen, the opening, gets progressively narrower.

So the organ slowly starves of blood over years.

Exactly.

It's chronic ischemia.

The kidney or heart or lung just slowly shrinks and scars over time until it stops working.

So to summarize, hyperacute is minutes because of preformed antibodies.

Acute is months from a new T cell or antibody attack, and chronic is years due to vascular fibrosis.

You've got it.

That's the framework.

We have covered an immense amount of ground today.

I mean, from the explosion of an allergy to the slow starvation of a transplanted organ.

Let's do a rapid fire summary to lock all this in before we go.

Let's do it.

Go.

Type 1, IgE and mast cells.

Think anaphylaxis.

Type 2, antibodies versus cells.

The sniper.

Type 3, immune complexes getting stuck.

The shrapnel.

And type 4, T cells.

The delayed infantry.

Autoimmunity.

SLE.

ANNA and anti -DSDNA.

The butterfly rash.

Kidney failure.

Sjogren.

Dry eyes, dry mouth.

Scleroderma.

Hard skin.

Diffuse versus the more benign Crest syndrome.

Primary deficiency.

Brutal.

No B cells in boys.

D George.

No thymus.

No parathyroids.

SCID.

Total failure from ADA deficiency or a cytokine receptor problem.

Wiskud Aldrich.

Eczema, bleeding, and infections.

AIDS.

The key is a CD4 count under 200.

Entry is via GP120 and a co -receptor like CCR5.

And the clinical picture is dominated by opportunistic infections.

And transplant.

Hypercute is minutes.

Acute is months.

Chronic is years.

So what does this all mean?

We've looked at the system exploding, attacking itself, having missing parts, and getting hijacked by a virus.

What's the big picture here?

You know, if we connect all these dots, it makes you realize that health is just this incredibly precarious balance.

We think of the immune system as a soldier, but it's really a diplomat and a demolition expert wrapped into one.

It has to perfectly tolerate self while being absolutely ruthless and destroying non -self.

And if that balance tips even a fraction of a degree in either direction?

Exactly.

Little too much aggression and you get hypersensitivity and autoimmunity.

The demolition expert starts blowing up your own house.

Little too little aggression or you're missing your weapons and you get immunodecision to your aides and the outside world comes in and destroys you.

The study of pathology, really, is the study of that fragile, fragile equilibrium.

A fragile equilibrium indeed.

Thank you so much for diving deep with us today.

Always a pleasure.

This has been the Last Minute Lecture Team.

Thanks for listening.