Chapter 6: Diseases of the Immune System

The text thoroughly outlines the cellular and molecular components of the innate immune response, including epithelial barriers, phagocytes, natural killer cells, and pattern recognition receptors (such as Toll-like and NOD-like receptors), alongside the highly specific adaptive immune response governed by T and B lymphocytes, antigen-presenting cells, and the major histocompatibility complex (MHC). Building upon these fundamental concepts, the chapter systematically categorizes immunologically mediated tissue injury into four distinct hypersensitivity reactions: immediate, IgE-mediated responses (Type I) typical of allergies and anaphylaxis; antibody-mediated cellular dysfunction and destruction (Type II); immune complex-mediated inflammation (Type III) seen in conditions like serum sickness; and T cell-mediated cellular cytotoxicity and delayed-type hypersensitivity (Type IV). A profound emphasis is placed on the breakdown of central and peripheral immunologic tolerance, which paves the way for autoimmune diseases. The etiology and morphological hallmarks of prominent systemic autoimmune conditions are detailed, notably Systemic Lupus Erythematosus (SLE), driven by antinuclear antibodies and immune complex deposition; Sjögren syndrome, characterized by immune-mediated destruction of exocrine glands; and Systemic Sclerosis (scleroderma), marked by progressive tissue fibrosis and microvascular damage. Furthermore, the chapter delves into the immunological mechanisms underlying tissue transplant rejection, differentiating between hyperacute, acute, and chronic rejection pathways, as well as graft-versus-host disease (GVHD). The spectrum of immunodeficiency diseases is heavily analyzed, distinguishing between congenital, primary defects in immune cell maturation or activation—such as Severe Combined Immunodeficiency (SCID), X-linked agammaglobulinemia, and DiGeorge syndrome—and secondary, acquired immune failures. Chief among the latter is a rigorous clinical and pathological review of HIV infection and AIDS, detailing the viral pathogenesis, CD4+ helper T-cell depletion, and the resulting susceptibility to opportunistic infections and neoplasms like Kaposi sarcoma. Finally, the chapter concludes with a detailed examination of amyloidosis, explaining how the extracellular deposition of abnormal, misfolded fibrillary proteins—whether amyloid light chain (AL), amyloid-associated (AA), or variant transthyretin (ATTR)—leads to insidious and progressive multi-organ dysfunction that is classically identified through Congo red staining and apple-green birefringence.