Chapter 11: Serology Techniques: Past, Current, and Future

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome to the Deep Dive.

Today, we're really getting into the weeds with a chapter from Forensic Biology, second edition by Richard Lye.

We're focusing on forensic serology.

That's the science that deals with identifying biological fluids before you even think about DNA.

It's foundational stuff.

Absolutely foundational.

And, you know, it's critical to remember why we do this.

These biological stains, they offer two key pieces of information.

First, what we call class characteristics.

Simply put, what is this fluid?

Is it blood, semen, saliva, or maybe just, you know, spilled coffee?

Okay, step one, identify the substance.

Exactly.

Then comes step two, the individual characteristics.

That's where we ask, who did it come from?

And that usually means moving on to DNA profiling, like STR analysis.

So serology is kind of the gatekeeper.

It tells you if the sample is even relevant before you run expensive DNA tests.

Precisely.

If you find a reddish -brown stain, you can't just assume it's blood and run DNA.

You have to confirm it's human blood first for that DNA profile to mean anything in court.

Right.

So, okay, let's unpack that process.

Imagine you're at a crime scene.

You see a suspicious stain.

How do you go from just looking at it to knowing for sure what it is?

It's a structured workflow.

Start simple.

Visual examination.

Does it look like blood or semen or whatever?

But looks can be deceiving.

So the first real test is usually a presumptive assay.

Think of these as quick and dirty screens.

They're designed to be super sensitive, fast and easy to use, often right there at the scene.

Sensitive meaning they can detect really tiny amounts.

Yes.

Very sensitive.

And here's the key thing about presumptive tests.

A negative result is pretty reliable.

If it says no, the fluid is almost certainly not there.

Okay.

So negative means stop looking here.

Right.

But a positive result.

Well, that only suggests a possibility.

It doesn't confirm anything.

Well, why not?

If it's positive, isn't that good?

It's a good indicator.

But these tests lack high specificity.

That means other things might trigger a positive result.

Certain chemicals, plant materials, rust even.

A false positive is a real risk.

Ah, okay.

So a positive presumptive test just flags it for a more rigorous testing back at the lab.

Exactly.

That's where confirmatory assays come in.

These are the tests designed for high specificity.

They tell you with a much higher degree of certainty, yes, this is human blood or yes, this is semen.

And historically, a lot of these tests, both presumptive and confirmatory, rely on a really basic biological interaction, right?

Antigens and antibodies.

That's the core mechanism for many.

Yes.

The way an antibody specifically recognizes and binds to its target antigen is fundamental.

How we detect that binding is what separates the main types of assays.

Let's talk about those types.

Broadly, you have primary binding assays.

These are generally more sensitive because they detect that very first interaction, a single antibody binding site locking onto a single antigen epitope, think ELISA.

And the other type.

Secondary binding assays.

These are usually less sensitive.

They rely on seeing a downstream effect of that binding, like visible clumping, agglutination, or precipitation line forming.

You need a lot more antigen and antibody for that to happen visibly.

Okay.

Let's dive into the primary assays first because sensitivity is key in forensics.

You mentioned ELISA enzyme linked immunosorbent assay.

That sounds pretty high tech.

It is, and it's been a workhorse.

It's great for detecting specific protein markers, like PSA for semen or amylase for saliva.

How does it actually work?

Can you sort of walk us through the common type, the antibody sandwich, ELISA?

Sure.

Imagine a small plastic well.

First, you coat the bottom with the capture antibody.

Then you add your sample extract.

If the target antigen, say PSA, is present, it binds to that capture antibody.

Okay.

So it's stuck there.

Right.

Then you wash away everything else.

Next, you add a second antibody that also recognizes PSA, but this one has an enzyme attached to it, like a little flag.

This binds to the captured PSA forming the sandwich,

antigen antibody.

The sandwich.

Got it.

Finally, you add a chemical substrate.

The enzyme flag reacts with it, producing a color.

The more antigen you had, the stronger the color signal.

It's quantitative, or at least semi -quantitative.

Very precise, but sounds like it needs lab equipment, not something you do at the scene.

Definitely a lab technique.

For field use, or just rapid screening in the lab, you have immunochromatographic assays.

Most people know these as lateral flow tests, think pregnancy tests or those rapid COVID tests.

Wow, the test strips.

Like the ABA card hematase for blood or RSID tests for saliva or semen.

Exactly those.

They use the same antigen antibody principle, but embed it all onto a membrane strip.

It's designed for speed and simplicity.

How does the strip work?

You apply the sample extract to one end.

If the antigen is present, it binds to a colored antibody that's already on the strip, but is mobile.

This complex then flows along the membrane by capillary action.

It flows towards two lines.

The first is the test zone, where another antibody is immobilized, stuck in place.

This one captures the colored antigen antibody complex, forming a visible line if the antigen is present.

And the second line?

That's the control zone.

It has an immobilized antibody that just captures some of the colored antibody, regardless of whether antigen was present.

It must show up for the test to be valid.

It proves the sample flowed correctly and the reagents worked.

Okay.

Test line plus control line equals positive.

Control line only equals negative.

No control line means the test failed.

Simple enough.

But you mentioned a potential issue earlier, something about too much antigen.

Ah yes, the high dose hook effect.

This is a really critical artifact, especially with these rapid strip tests.

It can cause a false negative, which is obviously problematic.

A false negative?

How does too much antigen cause the test to fail?

That seems counterintuitive.

It does, but here's what happens.

If your sample is incredibly concentrated, like a neat semen stain, you have a massive amount of antigen.

This flood of antigen does two things.

First, it saturates all the mobile colored antibodies.

Second, the free antigen molecules flow down the strip alongside the saturated colored antibodies.

When they reach the test zone… Let me guess.

The free antigen gets there first and blocks the sites.

Not quite blocks, but it competes.

There are only so many immobilized capture antibodies on the test line.

The free antigen and the colored antibody antigen complexes are all competing for those limited spots.

Because there's so much free antigen competing, not enough of the colored complexes can bind to the test line to form a visible signal.

The line just doesn't show up, or is very faint, even though the sample is loaded with antigen.

Wow.

So a strong sample can look negative?

That's, yeah, a nightmare scenario.

How do you avoid that?

The standard solution is dilution.

If you suspect a high concentration or get an ambiguous result, you dilute the sample extract and run the test again.

Often that resolves the issue.

Good to know.

Okay, before we completely leave these immunoassays, what about those older secondary binding methods you mentioned?

The ones based on visible clumping or precipitation.

Right, the secondary binding assays, things like immuno -diffusion.

Maybe you've heard of the Outer Loney test.

You put antigen and antibody in separate wells in an agar gel, and they diffuse towards each other.

Where they meet in optimal proportions, you see a visible white line of precipitate.

Okay, literally watching proteins fall out of solution.

Pretty much.

Or you have agglutination reactions, like classic ABO blood typing.

Antibodies cause red blood cells carrying the corresponding antigen to clump together visibly.

But you said these are less sensitive.

Much less sensitive, and often slower, needing more sample.

They were foundational, used a lot for species identification or blood grouping historically, but they've largely been superseded by more sensitive primary assays like ELISA or the molecular methods we're about to discuss.

Right, that makes sense.

The need for more sensitivity, more specificity, and the ability to work with tiny or degraded samples really push the field towards molecular biology.

Let's shift gears there.

Where do we start?

Maybe DNA and methylation?

An excellent starting point.

This is really clever.

We're not looking at the DNA sequence itself for identity here, but rather for chemical modifications on the DNA that tell us about the tissue source.

Modifications?

Like what?

Specifically, methyl groups added to cytosine bases,

usually where a cytosine is followed by a guanine, a CPG site.

The pattern of which CKG sites are methylated differs significantly between different body tissues.

We call these tissue -specific differently methylated regions, or TDMRs.

For example, certain regions are consistently unmethylated, or hypomethylated, in sperm cells compared to, say, blood cells or skin cells.

So it's like a chemical barcode for the tissue type written onto the DNA itself.

How do you read that barcode?

A common technique is bisulfite sequencing.

Sodium bisulfite is a chemical that converts unmethylated cytosines into uracil, which then gets read as a thymine after PCR amplification.

Methylated cytosines, however, are protected.

They stay as cytosines.

So you sequence the treated DNA, compare it to the original sequence, and wherever you see a C change to a T, you know it wasn't methylated.

Where it stays a C, it was methylated.

Precisely.

It gives you a very precise map of methylation patterns, allowing you to identify the tissue source with high confidence, even from challenging samples.

Very cool.

But methylation isn't the only molecular game in town.

RNA is also used, right?

Yes.

RNA analysis offers another window.

The idea here is to look at gene expression.

Different tissues actively express different genes.

So you look for messenger RNA, mRNA transcripts that are specific to certain fluids.

Exactly.

For example, you might look for KLK3 mRNA, which codes for PSA, as a marker for semen, or LS2 mRNA for blood.

If you detect that specific mRNA, it strongly indicates the presence of that fluid.

But isn't mRNA notoriously fragile?

That's the major drawback.

mRNA degrades very easily, especially in forensic samples exposed to the environment.

It can be tough to get reliable results from degraded evidence.

Which led scientists to look for something tougher.

Indeed.

And they found microRNA.

These are tiny RNA molecules, just 21 to 23 nucleotides long.

Their small size makes them much more stable and resistant to degradation compared to mRNA.

And do they also show tissue -specific patterns?

They do.

Different bodily fluids have distinct mRNA profiles.

Different sets of mRNAs are present or abundant.

So by detecting these specific, stable mRNAs, you can identify the fluid type even in samples where mRNA might be too degraded.

It's a very promising area.

Okay, we've covered DNA modifications and RNA expression.

What about looking directly at the proteins themselves?

You mentioned mass spectrometry earlier.

Right, proteomics using mass spec.

Think of a mass spectrometer as an incredibly precise scale for molecules.

It measures the mass -to -charge ratio of ions.

How does that help identify, say, blood?

Well, we often use tandem mass spectrometry, MSMS.

And typically we use a bottom -up approach for complex forensic samples.

First, you take the protein mixture from your sample and use an enzyme, like trypsin, to chop the proteins into smaller pieces called peptides.

Okay, break it down first.

Then the mass spec measures the masses of these peptides.

It can even select a specific peptide ion, smash it into even smaller fragment ions, and measure their masses too.

That's the tandem MS part.

And that fragmentation pattern tells you the peptide sequence.

Essentially, yes.

By analyzing the masses of the fragments, you can deduce the amino acid sequence of the peptide.

Then you search that sequence against protein databases.

If you find matches to known proteins, like hemoglobin for blood or PSA for semen, you've identified the fluid.

That sounds powerful, especially for complex mixtures, maybe.

It can be, yes.

Bottom -up proteomics is well -suited for analyzing the complex mixtures you often find in forensic evidence.

Okay, before we wrap up with techniques, there's one more biological source you mentioned in the reading, something maybe unexpected, our own microbes.

Ah, yes, microbial DNA analysis.

A fascinating angle.

Our bodies are ecosystems teaming with bacteria and other microbes, the microbiota.

We actually have more microbial cells than human cells.

Then the idea is that different parts of the body have different microbial communities.

Exactly.

The bacteria living in your mouth are very different from those in your gut or on your skin or in vaginal secretions.

For example, streptococcus species are abundant in saliva, while lactobacillus species often dominate the vaginal microbiome.

So if you find DNA from those specific bacteria in a stain, it strongly suggests the origin of the fluid.

You could detect these signature microbes by targeting specific bacterial DNA markers, like the 16S rRNA gene, which is commonly used for bacterial identification.

It's another way to infer the tissue source.

That's incredibly clever, using the passengers to identify the vehicles, so to speak.

Now, one big theme cutting across all of this is evidence consumption.

Elysa, DNA extraction, RNA analysis, they all use up some of the sample.

Which is a major concern, especially when you have only a tiny speck of material.

You might have just enough for one analysis.

Do you use it for serology to confirm it's blood, or do you go straight for DNA and hope it's relevant?

That's the dilemma, which leads us to the need for non -destructive assays, techniques that can identify the fluid without consuming the sample.

Absolutely critical.

And spectroscopy offers promising solutions here.

Two main techniques are gaining traction,

fluorescent spectroscopy and roma spectroscopy.

How does fluorescence work for this?

Different biological molecules, proteins, nucleic acids, metabolites found in bodily fluids absorb light and then emit light at a different wavelength.

They fluoresce.

The key is that different fluids have characteristic fluorescence signatures, or spectra, based on their composition.

So you shine a light on the stain, measure the emitted light spectrum, and match it to known fluid profiles, and it doesn't damage the sample.

Correct.

It's rapid, reagent -free, and non -contact.

You get an identification based on inherent optical properties of the stain itself.

In Raman spectroscopy, you described it as a molecular signature.

It is.

Raman spectroscopy involves shining a laser on the sample and measuring how the light scatters.

Most light scatters elastically, same energy, but a tiny fraction scatters inelastically, meaning it gains or loses energy due to interactions with vibrating molecules in the sample.

That energy shift is specific to the molecular bonds present.

The resulting Raman spectrum is like a detailed molecular fingerprint of the substance.

Different fluids, blood, semen, saliva, urine, have unique Raman signatures.

And it's also non -destructive.

Highly non -destructive.

It requires tiny sample amounts, picoliters, even femtoliters, uses low laser power, and doesn't consume anything.

You can analyze the stain directly on the evidence item.

So both fluorescence and Raman offered the potential to identify the fluid first, confirm its relevance, and then proceed with DNA analysis on the very same untouched spot.

That's the goal.

Maximizing the information gained while preserving that precious evidence for downstream DNA profiling.

It's about getting the most out of often limited material.

So to sort of wrap this up, we started with that core idea.

Forensic serology establishes the what's the class characteristic before we get to the who the individual characteristic via DNA.

We've traced the evolution from older methods relying on visible precipitation or agglutination, which needed quite a bit of sample.

Through to highly sensitive amino acids like ELISA and rapid strip tests that we have to watch out for things like the hook effect.

Then into the molecular age,

using DNA methylation patterns or specific RNA molecules like the stable marinades as highly specific tissue barcodes.

And even leveraging proteomics with mass spec or the body's own microbial signatures.

And now the push towards non -destructive techniques like Raymond and fluorescent

spectroscopy, aiming to identify fluids instantly without consuming any evidence, preserving it all for that crucial DNA profile.

It really highlights how the field constantly seeks more specificity, more sensitivity, and better ways to preserve evidence.

So here's a final thought for you the listener to mull over.

We've talked about these incredibly sensitive and specific new molecular methods,

DNA methylation, they seem almost definitive.

But thinking about real world casework.

What limitations do you think still exist when forensic scientists face really complex samples, maybe mixtures of multiple fluids or stains that are heavily degraded by time or environment?

And looking ahead, how might they overcome those remaining hurdles?

What's the next innovation likely to be?

Something to consider.

Thank you for joining us for this deep dive into the fascinating world of forensic serology.

We hope this gives you a solid understanding of how science identifies those silent witnesses at crime scenes.

Catch you next time on the deep dive.