Chapter 33: Adaptive Immunity – Specific Defenses & Memory

Adaptive immunity, an integrated host defense system working alongside innate immunity, relies on highly specific recognition, diversity, and long-term memory to differentiate between self and non-self substances. This defense operates through two branches: humoral immunity, involving B cells and circulating antibodies, and cellular immunity, mediated by T cells, including cytotoxic T lymphocytes (CTLs). Immunogens, or antigens, are typically large, complex molecules that possess specific antigenic determinant sites called epitopes, although smaller molecules called haptens can become immunogenic when bound to carrier proteins. Antigen presentation is crucial, utilizing the Major Histocompatibility Complex (MHC). MHC Class I molecules, found on all nucleated cells, present endogenous antigens to CD8+ T cells (CTLs), signaling infected or cancerous status, while MHC Class II molecules on antigen-presenting cells (APCs) present exogenous antigens to CD4+ T helper cells. T cells undergo thymic selection and require three signals—MHC/TCR binding (Signal 1), costimulation (Signal 2), and cytokine direction (Signal 3)—to activate and differentiate into subsets like TH1, TH2, TH17, or regulatory T cells (Tregs). Activated B cells produce five classes of immunoglobulin (IgG, IgA, IgM, IgD, IgE), which possess Fab (antigen-binding) and Fc (cell-binding) regions and achieve immense diversity through genetic rearrangement processes like combinatorial joining and somatic hypermutation. Antibodies function by neutralization, opsonization, complement activation, and formation of immune complexes. The resulting immunological memory ensures a rapid and intense secondary response upon reexposure. The chapter also explores how immunity can be naturally or artificially acquired, either actively (via infection or vaccination) or passively (via transferred antibodies), as well as immune system malfunctions, including hypersensitivities (Types I-IV), the loss of tolerance leading to autoimmune diseases, tissue rejection (host-versus-graft), and immunodeficiencies. Modern immunotherapies, such as immunomodulation and adoptive cellular therapy (ACT) utilizing CAR T cells, demonstrate the system’s potential in treating cancers like melanoma and leukemia.