Chapter 19: Disorders Associated with the Immune System

Type I (anaphylactic) reactions involve IgE antibodies triggering mast cells and basophils to release histamine, leading to conditions like hay fever, asthma, and systemic anaphylaxis. Type II (cytotoxic) reactions occur when antibodies bind to antigens on host cells, activating complement and leading to cell lysis, such as in transfusion reactions or hemolytic disease of the newborn. Type III (immune complex) reactions involve antigen-antibody complexes depositing in tissues and causing inflammation, exemplified by glomerulonephritis. Type IV (delayed cell-mediated) reactions are T-cell-driven and include contact dermatitis (like poison ivy) and tissue rejection. The chapter also covers autoimmune diseases, where the immune system loses self-tolerance and attacks the body’s own tissues. These are classified based on the immune mechanism involved—cytotoxic (e.g., Graves' disease), immune complex (e.g., systemic lupus erythematosus), and cell-mediated (e.g., multiple sclerosis). The chapter discusses the HLA system (human leukocyte antigen) and how certain HLA types increase susceptibility to specific autoimmune conditions. Immunodeficiencies are next, split into congenital (primary) immunodeficiencies like severe combined immunodeficiency (SCID) and acquired (secondary) types, most notably AIDS. The chapter explains how HIV infects CD4⁺ T cells, progresses through clinical stages, and ultimately compromises the immune system, leading to opportunistic infections and cancers. It describes how the virus is transmitted, how it evades the immune response through antigenic variation, and how antiretroviral therapies work to slow progression. By the end of the chapter, students understand the full range of immune system disorders—from overreactions to dangerous deficiencies—and how they are diagnosed, managed, and sometimes prevented. This knowledge is essential for grasping the clinical consequences of immune dysfunction and the basis of treatments ranging from allergy desensitization to immunosuppressive therapies.