Immunity

Immunity is the ability of an organism to resist or recover from infections, disease, or foreign substances. It encompasses innate and adaptive defense systems that work together to protect the body.

Innate immunity provides immediate, non-specific defense through physical barriers such as skin and mucous membranes; cellular responses via phagocytes and natural killer cells; humoral components like the complement system; and inflammatory processes that help contain infections.

Adaptive immunity develops more slowly but offers targeted and lasting defense. It relies on lymphocytes, including B cells that produce antibodies and T cells that mediate cellular immunity. Key features are specificity, diversity generated by gene rearrangement, and immunological memory that enhances responses upon subsequent exposures.

The immune system is organized into primary lymphoid organs, where lymphocytes mature (bone marrow and thymus), and secondary lymphoid organs (lymph nodes, spleen, and mucosa-associated lymphoid tissue) where responses are coordinated. Central and peripheral tolerance mechanisms help prevent reactions against self-antigens.

Immunity can be natural or artificial, and active or passive. Natural active immunity arises after infection, natural passive immunity transfers maternal antibodies to the fetus or infant. Artificial active immunity results from vaccination, while artificial passive immunity uses preformed antibodies or immune globulins to provide immediate, short-term protection.

Immunity can be compromised by immunodeficiency or misdirected by autoimmunity and hypersensitivity. A solid understanding of immune mechanisms underpins vaccines, immunotherapies, and strategies for preventing and managing infections.