Immunity is the capacity of an organism to recognise and resist harmful biological agents, foreign substances or abnormal cells through coordinated defensive mechanisms. In humans, immunity involves physical barriers, innate immune mechanisms and adaptive immune mechanisms. Immunity can be present naturally through inherited or continuously functioning defence mechanisms, or it can be acquired through exposure, vaccination, maternal antibody transfer or medical interventions. Adaptive immunity is particularly notable because it can produce specificity and memory.
Immunity does not necessarily mean complete prevention of infection. A person may possess immunity that reduces the probability of infection, lowers pathogen replication, decreases disease severity or shortens the duration of illness. The degree and duration of protection depend on the pathogen, immune mechanism, exposure route, previous encounters and characteristics of the individual and microorganism.
Innate immunity is the body's rapid, broadly acting defence system. It includes skin, mucosal barriers, antimicrobial substances, normal microbial communities, complement proteins, phagocytic cells, natural killer cells and inflammatory responses. Innate mechanisms can respond within minutes or hours and generally recognise conserved molecular patterns associated with pathogens or tissue damage. Adaptive immunity is more specific and is mediated principally by B and T lymphocytes.
B cells can generate antibodies, while T cells provide cellular functions including coordination of immune responses and destruction of certain infected or abnormal cells. Adaptive responses initially take longer to develop because specific lymphocytes must be activated and expanded, but they can produce memory cells that alter responses to subsequent exposure.
Active immunity develops when an individual's own immune system is stimulated to generate a response. Infection can produce naturally acquired active immunity, while vaccination produces artificially acquired active immunity. Passive immunity occurs when ready made antibodies are transferred to an individual rather than generated by that individual's immune system in response to the immediate exposure. Maternal antibodies can provide passive protection to infants, and medically administered antibodies can provide temporary protection or treatment in particular circumstances.
Passive immunity generally acts more rapidly than active immunity but does not usually generate the same durable immunological memory. The distinction between active and passive immunity is therefore based on the source of protective immune components and has important implications for onset and duration of protection.
Immunological memory is a central feature of adaptive immunity. During an initial exposure, selected B and T lymphocytes expand and differentiate into effector and memory populations. Memory B cells can contribute to faster antibody responses upon subsequent exposure, while memory T cells can respond more rapidly to familiar antigenic structures. Long lived plasma cells can continue producing antibodies after the original infection or vaccination has ended. The strength and duration of memory vary substantially among pathogens and vaccines. Some pathogens produce durable immunity after infection, while others can reinfect individuals because immunity declines, the pathogen changes its antigens or protective immune responses do not completely prevent reinfection. Immunity is therefore dynamic rather than a permanent binary state.
Immunity can be highly specific. Antibodies and lymphocyte receptors recognise particular molecular structures, allowing adaptive responses to distinguish among many different antigens. However, specificity does not always mean absolute exclusivity because related antigens can sometimes cross react. Protective immunity may also involve several mechanisms simultaneously. Antibodies can neutralise extracellular pathogens, while cytotoxic T cells can destroy infected cells and helper T cells can coordinate other immune functions. Innate mechanisms such as complement and phagocytosis can work alongside adaptive responses. Effective immunity is therefore an integrated network rather than a single antibody concentration or one measurable immune component.
Immunity must be regulated to avoid damaging healthy tissues. Tolerance mechanisms suppress many potentially harmful responses against self molecules. Regulatory T cells and inhibitory signalling pathways help maintain this balance, while controlled resolution limits unnecessary inflammation after a threat has been contained. When tolerance fails, autoimmune reactions can occur. When immune responses against harmless substances become excessive, allergic disease can develop. Conversely, inadequate immune defence can result from genetic defects, acquired conditions, certain medicines or other factors and can increase susceptibility to infection. These possibilities demonstrate that healthy immunity depends on appropriate specificity, strength and regulation rather than maximum immune activity.
Immunity varies between individuals and across the lifespan. Genetic variation can influence immune receptors, cytokine signalling and susceptibility to particular infections. Age affects immune development and later immune function, while previous infections and vaccinations establish different immune histories among individuals. Nutrition, sleep, stress physiology, environmental exposures and the microbiome can also influence immune processes, although effects differ among circumstances. Pathogens themselves evolve, and immune protection can be reduced when antigenic characteristics change sufficiently to escape existing recognition. This is one reason why some vaccines require updated formulations or repeated doses. Immunity should therefore be understood as a changing interaction between host biology, previous exposure and the evolving characteristics of infectious agents.
Immunity is distinct from resistance, tolerance and complete sterilising protection, although these concepts overlap. Resistance refers broadly to mechanisms that reduce pathogen burden or prevent infection, while tolerance refers to mechanisms that limit damage caused by infection without necessarily reducing pathogen numbers. Some immune responses provide strong resistance, while others mainly reduce disease severity. Vaccination can establish protective immune memory without requiring the individual to experience the full disease.
Measuring immunity may involve antibody concentrations, cellular responses, functional neutralisation assays or epidemiological evidence, and no single measurement captures every form of protection. Modern immunology therefore treats immunity as a collection of interacting mechanisms whose effectiveness depends on pathogen biology, exposure conditions, immune history and the balance between defence and tissue preservation.
