The immune system is a distributed biological defence system that protects the body from infectious organisms, abnormal cells and potentially harmful substances while also maintaining tolerance toward the body's own healthy tissues. It consists of physical barriers, specialised cells, tissues, organs, soluble proteins, signalling molecules and highly regulated cellular processes. The skin, mucous membranes, mucus, cilia, stomach acidity, antimicrobial substances and normal microbiota form important first lines of defence.
If potentially harmful material crosses these barriers, innate immune mechanisms respond rapidly through processes such as recognition, inflammation, complement activation and phagocytosis. Adaptive immune mechanisms then provide highly specific responses involving lymphocytes, antibodies and immunological memory. The immune system does not operate as a single organ or isolated pathway. It is a network in which cells communicate continuously, recognise molecular patterns, alter their behaviour and coordinate responses according to the type, location and severity of a threat.
Immunity begins with recognition. Immune cells possess receptors capable of detecting molecular features associated with pathogens or cellular damage. Innate immune receptors recognise conserved patterns found in groups of microbes, while adaptive lymphocyte receptors recognise highly specific molecular structures. Cells such as macrophages, dendritic cells and neutrophils are important components of innate defence. Dendritic cells can capture material from pathogens, process it and present antigen fragments to T lymphocytes, creating an important connection between innate and adaptive immunity.
Pattern recognition receptors can detect structures such as microbial nucleic acids, bacterial cell wall components and other pathogen associated molecular patterns. Damaged cells can also release molecules associated with tissue injury. Recognition activates signalling pathways that alter gene expression, stimulate cytokine production, recruit immune cells and initiate mechanisms designed to contain or eliminate the threat.
The innate immune system provides rapid defence and includes physical barriers, antimicrobial molecules, complement proteins, phagocytic cells, natural killer cells and inflammatory mechanisms. Neutrophils can rapidly migrate toward sites of infection and engulf microorganisms. Macrophages consume pathogens and cellular debris while also producing signalling molecules and contributing to tissue repair. Natural killer cells can identify and destroy certain infected or abnormal cells, particularly when normal patterns of cellular recognition have been disrupted.
Complement is a network of plasma proteins that can enhance inflammation, mark pathogens for destruction and, in some circumstances, directly damage microbial membranes. Innate immunity is comparatively rapid and recognises broad molecular patterns rather than the enormous range of individual antigens recognised by adaptive lymphocytes. It provides immediate protection while also helping determine the character and location of subsequent adaptive responses.
Adaptive immunity involves B lymphocytes and T lymphocytes and is distinguished by highly specific antigen recognition and immunological memory. B cells can differentiate into plasma cells that produce antibodies directed against particular antigens. T cells have several functional categories. Helper T cells coordinate immune responses through signalling and interactions with other immune cells, while cytotoxic T cells can kill infected or abnormal cells. Regulatory T cells help restrain excessive immune activation and contribute to tolerance.
During an initial immune response, selected lymphocytes undergo clonal expansion, producing populations of cells with receptors suited to the recognised antigen. Some become effector cells that act immediately, while others become memory cells. On later exposure to the same or sufficiently similar antigen, memory cells can respond more rapidly and strongly, forming the basis of long lasting adaptive immunity.
Communication is essential to immune function. Cytokines are signalling proteins that influence the activity, growth, movement and differentiation of immune cells. Chemokines are signalling molecules particularly important for directing cell migration toward particular tissues. Antigen presentation allows information about encountered molecules to be communicated between innate antigen presenting cells and adaptive lymphocytes. Major histocompatibility complex molecules display peptide fragments on cell surfaces for recognition by T cell receptors.
Different classes of immune cells respond to different combinations of signals, which helps prevent inappropriate activation. Immune regulation is equally important because uncontrolled immune activity can damage healthy tissues. Anti inflammatory signals, regulatory cells, programmed cell death and other mechanisms help terminate responses after threats have been controlled and promote restoration of normal tissue function.
The immune system operates throughout the body rather than being confined to a single location. Bone marrow produces many blood and immune cells, while the thymus is particularly important for T cell maturation. Lymph nodes filter lymph and provide environments where antigen presenting cells and lymphocytes can interact. The spleen monitors blood and contributes to immune responses against blood borne organisms. Mucosal immune tissues are located at surfaces exposed to the external environment, including the respiratory and gastrointestinal tracts. Immune cells also reside in tissues and can respond locally. Blood and lymphatic vessels provide routes through which immune cells and soluble factors circulate. This organisation allows the immune system to detect threats at different anatomical sites while coordinating local and systemic responses.
Immune responses must distinguish harmful targets from healthy self tissues. During development, lymphocytes undergo selection processes that remove or control many cells capable of strongly recognising self molecules. Peripheral tolerance mechanisms provide additional safeguards after immune cells mature. These include regulatory lymphocytes, inhibitory signalling pathways and other mechanisms that suppress inappropriate activation. Tolerance is not absolute, because immune recognition can sometimes become misdirected, contributing to autoimmune disease.
Immune responses can also become excessive or disproportionate, producing tissue damage through severe inflammation or allergic reactions. Conversely, inadequate immune function can increase susceptibility to infections. The effectiveness of immunity therefore depends not simply on having a powerful response but on achieving appropriate recognition, activation, targeting, containment and termination.
Immune function is shaped by age, genetics, previous infections, vaccination, nutrition, microbiota, environmental exposures and underlying physiological conditions. The immune system changes throughout life, with immune development occurring from infancy and many aspects of immune function changing during ageing. The microbiome interacts with immune development and regulation, particularly at mucosal surfaces.
Immunological memory means that previous exposure can influence subsequent responses, but protection is not necessarily permanent because pathogens can change their antigens, immunity can decline and different pathogens require different forms of defence. Overall, immune function can be understood as an integrated sequence involving barriers, recognition, signalling, cellular recruitment, pathogen elimination, tissue repair and memory. The system succeeds when it removes threats while limiting unnecessary damage to the host.
