The immune system defends the body against pathogens (bacteria, viruses, fungi, parasites), eliminates damaged or abnormal cells, and monitors the body’s own tissues for signs of malignancy. It is not located in a single organ — it is distributed throughout the body in the blood, lymph, and specialized tissues.
Innate immunity
The innate immune system provides immediate, non-specific defense — it responds the same way to any threat, without memory of previous encounters.
Physical barriers — the first line of defense:
- Skin — a continuous barrier of keratinized epithelial tissue that physically blocks pathogen entry. Breaks in skin (wounds, burns, IV access) are infection risks precisely because they breach this barrier.
- Mucous membranes — line the respiratory, gastrointestinal, and urogenital tracts. Mucus traps pathogens; cilia in the airways sweep mucus and trapped particles upward toward the pharynx (the mucociliary escalator).
- Chemical defenses — stomach acid kills ingested pathogens; lysozyme in tears and saliva destroys bacterial cell walls; sebum on skin creates an acidic, hostile environment for microbes.
Cellular innate immunity — when pathogens breach the barriers, innate immune cells respond:
- Neutrophils — the most abundant white blood cells; arrive first at sites of infection and kill pathogens by engulfing them (phagocytosis) and releasing toxic chemicals. Neutrophils are short-lived — they die at the site of infection, and their accumulation is what forms pus.
- Macrophages — larger phagocytic cells that engulf pathogens and dead cells. Macrophages also present pathogen fragments (antigens) to adaptive immune cells, bridging innate and adaptive immunity. Resident macrophages in specific tissues have specialized names: Kupffer cells (liver), microglia (brain and spinal cord), alveolar macrophages (lungs).
- Mast cells — release histamine and other inflammatory mediators when activated. Central to allergic reactions and the early stages of inflammation.
- Natural killer (NK) cells — destroy virus-infected cells and tumor cells by detecting the absence of normal surface markers. NK cells do not need prior exposure to recognize a threat — they detect “missing self” rather than “foreign.”
The inflammatory response — when innate immune cells detect a pathogen, they release inflammatory mediators (prostaglandins, histamine, cytokines) that produce the inflammatory response: increased blood flow, increased vascular permeability, and recruitment of more immune cells to the site. Inflammation is the innate immune system’s primary amplification mechanism.
Adaptive immunity
The adaptive immune system is slower to respond (days rather than minutes) but is specific to particular pathogens and develops memory — the basis of vaccination.
Lymphocytes — the cells of adaptive immunity:
- B cells — produce antibodies (immunoglobulins) — proteins that bind specifically to antigens on pathogen surfaces. Antibody binding neutralizes pathogens directly (blocking viral attachment to cells), marks them for phagocytosis (opsonization), and activates the complement system (a cascade of proteins that punch holes in pathogen membranes). After an infection resolves, some B cells persist as memory B cells, enabling a faster, stronger antibody response on re-exposure. This is the mechanism of vaccination: exposure to a harmless antigen (inactivated virus, viral protein, mRNA encoding a viral protein) generates memory B cells that produce protective antibodies if the actual pathogen is encountered.
- T cells — mature in the thymus and perform multiple functions:
- Helper T cells (CD4+) — coordinate the immune response by releasing cytokines that activate B cells, cytotoxic T cells, and macrophages. HIV destroys CD4+ T cells, which is why AIDS produces immunodeficiency — without helper T cells, the immune system cannot mount effective responses.
- Cytotoxic T cells (CD8+) — directly kill virus-infected cells and tumor cells by recognizing foreign antigens displayed on cell surfaces.
- Regulatory T cells — suppress immune responses, preventing excessive inflammation and autoimmune reactions. They are the immune system’s brake.
Antigen presentation — adaptive immune cells do not detect pathogens directly. Instead, antigen-presenting cells (macrophages, dendritic cells) engulf pathogens, process their proteins, and display antigen fragments on their surfaces using MHC (major histocompatibility complex) molecules. T cells recognize these MHC-antigen complexes. This is why organ transplant rejection occurs — the donor organ’s MHC molecules are recognized as foreign by the recipient’s T cells.
The lymphatic system
The lymphatic system is the immune system’s infrastructure:
- Lymph vessels — a network of vessels that collect interstitial fluid (fluid that has leaked from capillaries into tissues) and return it to the blood. Lymph fluid passes through lymph nodes before returning to the circulation.
- Lymph nodes — small, bean-shaped organs distributed along lymph vessels. Lymph nodes contain high concentrations of lymphocytes and macrophages. As lymph flows through, immune cells survey it for pathogens. Swollen lymph nodes during infection reflect active immune responses — lymphocytes are proliferating within the nodes.
- Spleen — filters blood (as lymph nodes filter lymph). Contains B cells and T cells that respond to blood-borne pathogens. Also removes old and damaged red blood cells.
- Thymus — where T cells mature and undergo selection (T cells that react to self-antigens are eliminated, preventing autoimmunity). The thymus is most active in childhood and involutes (shrinks) with age.
- Bone marrow — produces all blood cells, including the white blood cells of the immune system. B cells mature in bone marrow; T cells migrate to the thymus for maturation.
Immune dysfunction
Immunodeficiency — insufficient immune function. Can be primary (genetic — severe combined immunodeficiency, SCID) or acquired (HIV/AIDS, chemotherapy-induced, corticosteroid-induced). Immunodeficient individuals are vulnerable to infections that healthy immune systems control easily (opportunistic infections).
Autoimmune disease — the immune system attacks the body’s own tissues. The regulatory mechanisms that normally prevent self-reactivity (thymic selection, regulatory T cells, peripheral tolerance) fail, and immune cells treat self-antigens as foreign. Examples: rheumatoid arthritis (immune attack on joint tissue), type 1 diabetes (immune destruction of insulin-producing pancreatic cells), multiple sclerosis (immune attack on myelin sheaths of neurons), lupus (immune attack on multiple organ systems). Treatment involves immunosuppression — reducing immune activity with corticosteroids or biologic agents that target specific inflammatory cytokines.
Allergy and hypersensitivity — exaggerated immune responses to harmless substances (pollen, food proteins, animal dander). Allergic reactions involve IgE antibodies bound to mast cells; when the allergen cross-links IgE molecules, mast cells degranulate and release histamine, producing the symptoms of allergy (itching, swelling, bronchospasm). Anaphylaxis is a severe, systemic allergic reaction that can produce life-threatening airway obstruction and cardiovascular collapse.
Immunity and the broader vault
Roberto Esposito’s immunitas uses the biological concept of immunity as a political-philosophical framework. The immune system protects the body by distinguishing self from non-self — and destroys what it identifies as foreign. Autoimmune disease is what happens when this distinction fails and the defense mechanism attacks its own constituency. Esposito argues that political systems exhibit the same logic: the community’s protective mechanisms (borders, policing, public health measures) can become autoimmune — attacking the community’s own members in the name of protecting the community. The parallel is structural, not metaphorical.
Related terms
- Inflammation — the immune system’s primary amplification mechanism, and a driver of disease when chronic
- Cell — the units that immune cells defend and, in autoimmune disease, attack
- Homeostasis — the stable state that immune function maintains