immunitety

Immunity refers to the ability of an organism to resist infection and disease. The immune system protects the body from pathogens such as bacteria, viruses, fungi, and parasites, as well as from abnormal or cancerous cells. It operates through two complementary systems: innate and adaptive immunity.

Innate immunity provides immediate, general defenses. Physical barriers like the skin and mucous membranes prevent entry of microbes. If a pathogen breaches these barriers, innate mechanisms include phagocytic white blood cells (such as neutrophils and macrophages), natural killer cells, the complement system, and inflammatory responses. These responses are non-specific and do not confer long-lasting protection against a particular pathogen.

Adaptive immunity develops after exposure to specific antigens. It is highly specific and produces immunological memory. Humoral immunity is mediated by B lymphocytes, which generate antibodies that bind to antigens and mark pathogens for destruction. Cellular immunity is driven by T lymphocytes, including helper T cells that orchestrate responses and cytotoxic T cells that destroy infected or abnormal cells. The memory component enables swifter and stronger responses upon subsequent encounters with the same pathogen.

Immunity can be active or passive. Active immunity arises from the immune system’s encounter with a pathogen or a vaccine, leading to long-lasting protection. Passive immunity results from receiving antibodies produced by another person or animal, offering immediate but usually temporary protection.

Disorders of immunity include immunodeficiency (reduced immune function), autoimmune diseases (immune attacks on self), and hypersensitivities such as allergies. Population-level protection is enhanced by herd immunity, achieved when a sufficient proportion of the community is immune, often through vaccination.