Chapter 17: Adaptive Immunity: Specific Defenses of the Host

Adaptive Immunity: Specific Defenses of the Host begins by differentiating adaptive immunity from innate defenses, highlighting its ability to mount stronger responses upon repeated exposures through immunological memory. Adaptive immunity is divided into humoral immunity, mediated by B cells that produce antibodies, and cellular immunity, governed by T cells that attack infected cells directly or help regulate immune responses. The humoral immune response is detailed through the activation of B cells by antigens and helper T cells, leading to clonal expansion and production of antibodies—IgG, IgM, IgA, IgD, and IgE. The structure and functions of these immunoglobulins are explained, including neutralization, opsonization, agglutination, complement activation, and antibody-dependent cell-mediated cytotoxicity (ADCC). The chapter also covers the role of antigen-presenting cells (APCs), including dendritic cells and macrophages, which present antigens via major histocompatibility complex (MHC) molecules to T cells. CD4⁺ helper T cells (TH1, TH2, TH17) coordinate immune responses, while CD8⁺ cytotoxic T lymphocytes (CTLs) destroy virus-infected or abnormal cells. Regulatory T cells (Tregs) suppress excessive responses and maintain self-tolerance. Cytokines—chemical messengers like interleukins, interferons, and tumor necrosis factor—are discussed as essential signals that coordinate immune cell behavior. The chapter describes clonal selection, where only B and T cells specific to an antigen are activated and proliferate, generating memory cells for faster secondary responses. It distinguishes between T-dependent and T-independent antigens and explains class switching in immunoglobulin production. The four types of adaptive immunity—naturally acquired active, naturally acquired passive, artificially acquired active, and artificially acquired passive—are defined with examples such as infections, maternal antibodies, vaccines, and immunoglobulin injections. By integrating the molecular, cellular, and systemic elements of adaptive immunity, this chapter provides a foundational understanding of how the immune system protects the body through specificity, diversity, and memory.