Immunology

The first line of defense against pathogens, consisting of physical barriers (skin, mucous membranes), chemical barriers (stomach acid, antimicrobial peptides), and cellular components (neutrophils, macrophages, natural killer cells) that respond rapidly and nonspecifically to infection. Unlike adaptive immunity, innate immunity does not generate long-lasting immunological memory.

The branch of the immune system that generates highly specific responses to particular pathogens through the activation of T lymphocytes and B lymphocytes. Adaptive immunity develops more slowly than innate immunity but produces immunological memory, allowing the body to mount faster and stronger responses upon re-exposure to the same pathogen.

Y-shaped glycoproteins produced by plasma cells (differentiated B cells) that specifically recognize and bind to antigens on pathogens or foreign substances. Antibodies neutralize pathogens, opsonize them for phagocytosis, and activate the complement system. The five major classes are IgG, IgA, IgM, IgE, and IgD.

T cells recognize antigens only when they are presented on the surface of other cells by major histocompatibility complex (MHC) molecules. MHC class I presents intracellular antigens to CD8+ cytotoxic T cells, while MHC class II presents extracellular antigens to CD4+ helper T cells. This ensures that T cell responses are directed at infected or abnormal cells.

The principle that each lymphocyte bears receptors specific for a single antigen, and upon encountering that antigen, the lymphocyte is activated and proliferates to produce a clone of identical effector cells. This theory, proposed by Frank Macfarlane Burnet, explains how the immune system generates specific responses from a diverse repertoire of lymphocytes.

The state of immunological unresponsiveness to self-antigens, which prevents the immune system from attacking the body's own tissues. Central tolerance occurs during lymphocyte development in the thymus (T cells) and bone marrow (B cells), while peripheral tolerance involves mechanisms such as regulatory T cells and anergy in mature lymphocytes.

A cascade of over 30 plasma proteins that, when activated, enhance (complement) the ability of antibodies and phagocytes to clear pathogens. The complement system operates through three pathways (classical, lectin, and alternative) and mediates opsonization, inflammation, and direct lysis of pathogens through the membrane attack complex.

Small signaling proteins secreted by immune cells that regulate and coordinate immune responses. Cytokines include interleukins, interferons, tumor necrosis factors, and chemokines, each with specific roles in promoting inflammation, activating lymphocytes, directing cell migration, or resolving immune responses.

Vaccination introduces weakened, inactivated, or component-based antigens to prime the adaptive immune system without causing disease. This generates memory T cells and memory B cells that persist long after the initial response, enabling a rapid and robust secondary immune response upon subsequent exposure to the actual pathogen.

Inhibitory pathways that modulate the intensity and duration of immune responses to prevent excessive tissue damage and autoimmunity. Key checkpoint molecules include CTLA-4, PD-1, and their ligands, which deliver inhibitory signals to activated T cells. Cancer cells often exploit these checkpoints to evade immune destruction.