Immunity (medicine)

Immunity, in a medical context, describes the body's ability to resist or be unaffected by harmful agents, most commonly pathogens that cause infectious disease. This resistance can develop naturally or be induced through prior exposure or vaccination.

The immune system operates through two interconnected branches: innate and adaptive. Innate immunity, also called native immunity, is the body's first line of defense. It is a semi-specific, widely distributed system found in all multicellular animals (metazoans). It relies on genetically encoded pattern recognition rather than adapting to a specific prior infection, and its responses include inflammation and phagocytosis. This system helps maintain homeostasis and is critical for activating the adaptive immune response.

Adaptive, or acquired, immunity is mediated by antigen-specific lymphocytes. Unlike the innate system, it is highly specific to a particular pathogen and generates immunological memory. It includes both humoral and cell-mediated components. Adaptive immunity can be acquired naturally (through infection) or artificially (through deliberate actions like vaccination). It is further classified as active or passive. Active immunity results from exposure to a pathogen, prompting the body to produce its own antibodies. Passive immunity involves the transfer of antibodies or activated T-cells from an immune host—either naturally (via the placenta) or artificially—but is short-lived and often requires booster doses. While innate immunity recognizes broad patterns, adaptive immunity recognizes a much more diverse set of molecular patterns and is uniquely associated with memory of the pathogen.

Historically, humanity has long sought explanations for disease and immunity. Prehistoric cultures often attributed illness to supernatural forces or divine punishment. In Classical Greece, Hippocrates proposed that disease stemmed from an imbalance in the four humors (blood, phlegm, yellow bile, black bile). The first written hints of immunity may come from the Athenian historian Thucydides, who around 430 BC noted that survivors of the plague in Athens could care for the sick without fear, as they were never fatally attacked a second time.

Early attempts at active immunotherapy are attributed to Mithridates VI of Pontus (120–63 BC).