Chapter 20: Cancer: Key Concepts and Mechanisms

Central to adaptive immunity are lymphocytes—B cells and T cells—that recognize antigens via highly diverse receptors generated through V(D)J recombination. B cells produce antibodies (immunoglobulins), which can neutralize pathogens directly or tag them for destruction, while T cells express T cell receptors (TCRs) and are classified into helper T cells (CD4⁺) and cytotoxic T cells (CD8⁺). The chapter explains how antigen-presenting cells (APCs), such as dendritic cells, process and present peptides on major histocompatibility complex (MHC) molecules—Class I for CD8⁺ and Class II for CD4⁺ T cells. Clonal selection ensures that only lymphocytes with antigen-specific receptors expand upon encountering their cognate antigen, and clonal expansion produces effector and memory cells. The chapter covers the signaling cascades that follow receptor activation, the co-stimulatory molecules required for full activation, and the transcriptional programs driving differentiation into effector cell types. Immunological tolerance, which prevents self-reactivity, is achieved via central and peripheral mechanisms, including negative selection in the thymus and the action of regulatory T cells (Tregs). The chapter also examines immunoglobulin class switching, affinity maturation, and the role of cytokines in orchestrating immune responses. Finally, it discusses how adaptive immunity provides long-term protection via memory cells and is the basis of vaccination, while also noting how dysregulation can result in autoimmune diseases, allergies, and immunodeficiencies.