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Immune system

The immune system is the body's defence network: cells, tissues and proteins that recognise substances called antigens and respond to them. Innate immunity — barriers such as skin and mucus, plus fast-acting cells — is present from birth and acts within minutes to hours. Adaptive immunity, carried out by B and T lymphocytes, takes days to weeks to build up but targets specific antigens and keeps a memory, so a second encounter is met faster; vaccines use that memory without the illness itself.

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Info: This page explains how the immune system works. It is not medical advice: questions about your own health, symptoms or vaccinations are for a doctor, nurse or pharmacist.

Specificity, tolerance and memory

Innate immunity is antigen-independent and acts immediately or within hours, but it has no immunological memory. Adaptive immunity is antigen-dependent and antigen-specific, involves a lag before its maximal response, and generates memory that allows a more rapid and efficient response on re-exposure. Its key functions are distinguishing non-self from self antigens, eliminating specific pathogens or infected cells, and building that memory.

  • Active immunity follows natural infection or vaccination; passive immunity is the transfer of ready-made antibodies, such as maternal antibodies across the placenta.
  • Tolerance normally prevents responses to self antigens and harmless substances.
  • Dysregulation produces autoimmune disease, immunodeficiency or allergy.
Common misconception: More immune activity is not automatically better. Too much, too little or the wrong immune response causes immune system disorders, such as autoimmune disorders and allergies.
Full explanation — the complete reference version every reading depth is based on
Info: This page explains how the immune system works. It is not medical advice: questions about your own health, symptoms or vaccinations are for a doctor, nurse or pharmacist.

What the immune system does

The immune system protects the body by recognising antigens and responding to them. Antigens are substances — usually proteins — on the surfaces of cells, viruses, fungi and bacteria; toxins and other non-living things can be antigens too. Your own cells carry antigens as well, and the immune system normally learns to treat them as 'self' and leave them alone.

Two layers of defence

  • Innate immunity is present from birth and is not specific to one antigen. It includes barriers — skin, mucus, the cough reflex, enzymes in tears and skin oils, stomach acid — and cells such as neutrophils, the most abundant white blood cells, which engulf and digest pathogens.
  • Innate responses start within minutes or hours, but they have no memory.
  • Adaptive (acquired) immunity is specific to particular antigens and builds up over days or even weeks.
  • Adaptive immunity has memory, so the next response to the same antigen is faster and more effective.

Inflammation

When tissue is damaged, injured cells release chemicals such as histamine. Blood vessels leak fluid into the tissue, causing swelling that helps isolate the problem, and phagocytes are drawn in to engulf germs and dead or damaged cells.

B cells, T cells and antibodies

Lymphocytes are the white blood cells of adaptive immunity. B cells arise in the bone marrow; when activated, they become plasma cells that release antibodies. T cells also come from bone-marrow stem cells but mature in the thymus; some attack infected cells directly and others help direct the whole response. An antibody is a Y-shaped protein built from four chains — two heavy and two light. Antibodies can block the parts of a pathogen it uses to enter cells and can mark pathogens for phagocytes to destroy.

Memory and vaccines

After a first encounter with an antigen, memory cells remain. If the same pathogen turns up again, the antibody response is much faster and more effective. Vaccines use this: they contain weakened or inactive parts of a pathogen, weakened or reconstituted whole viruses or bacteria, or a DNA or RNA blueprint for an antigen, and they prompt an immune response without causing the disease. According to the World Health Organization, some vaccines need more than one dose to build long-lived antibodies and memory cells, and no single vaccine provides 100% protection.

When many people in a community are vaccinated, a pathogen struggles to spread because most people it meets are immune. This herd immunity also protects people who cannot be vaccinated, though not completely.

Worked example: passive immunity

Not all protection is made by your own body. Babies are born with antibodies that crossed the placenta from their mother; these are gone by somewhere between 6 and 12 months of age. Because the baby's body did not make them, they leave no memory cells behind — which is the key difference between passive immunity and the acquired immunity that follows infection or vaccination.

Common misconception: A common misconception is that the immune system attacks anything it has not seen before and nothing else. In fact it normally tolerates the body's own antigens and many harmless substances; disorders arise when it reacts against the body's own tissue (autoimmunity), responds too weakly (immunodeficiency), or reacts to something harmless (allergy).
Common misconception: Another misconception is that a vaccine gives you a mild case of the disease. WHO explains that a vaccine prompts the immune system to respond as it would to the real pathogen, but does not cause the disease in the person vaccinated.

Where this connects

The immune system is made of specialised cells. Its enormous range of antibodies comes from recombining and mutating a few hundred gene segments, so it is also a story about genes.

Assumptions and limits

  • Timings such as 'minutes or hours' and 'days or even weeks' are the ranges the sources give, not a fixed timetable.
  • Protection is never all-or-nothing: no single vaccine gives 100% protection, and herd immunity gives people who cannot be vaccinated substantial but incomplete protection.
  • This page describes how the system works in general; it cannot say how any one person's immune system will respond.

How we know

Antibodies are so specific that laboratories use them as measuring tools: methods such as radioimmunoassays rely on the antigen–antibody interaction to detect and measure substances. Evidence for vaccination comes from what happens to disease at population scale — after decades of polio vaccination through routine immunisation and mass campaigns, the African continent was certified free of wild poliovirus in August 2020.

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Sources and methodology

  • The immune system protects the body by recognising and responding to antigens — substances, usually proteins, on the surface of cells, viruses, fungi or bacteria, as well as non-living substances such as toxins. (awaiting scientific review)
  • Body cells also carry antigens, and the immune system learns to see these as normal and usually does not react against them. (awaiting scientific review)
  • Innate immunity is the non-specific defence system a person is born with; it includes barriers such as the skin, mucus, the cough reflex, enzymes in tears and skin oils, and stomach acid. (awaiting scientific review)
  • Innate immunity responds within minutes or hours and has no immunological memory, whereas adaptive immunity is antigen-specific and has memory that enables a more rapid and efficient response on later exposure to the same antigen. (awaiting scientific review)
  • The adaptive immune response takes days or even weeks to become established, much longer than the innate response. (awaiting scientific review)
  • B lymphocytes become plasma cells that produce antibodies, while T lymphocytes attack antigens directly and help control the immune response. (awaiting scientific review)
  • T cells derive from stem cells in the bone marrow and mature in the thymus, while B cells arise from stem cells in the bone marrow and leave it after maturing. (awaiting scientific review)
  • An antibody molecule is made of four polypeptides — two identical heavy chains and two identical light chains — that form a Y-shaped structure. (awaiting scientific review)
  • Antibodies can neutralise pathogens by blocking the sites they use to infect host cells, and can tag pathogens for destruction by phagocytes such as macrophages or neutrophils. (awaiting scientific review)
  • Secreted antibodies bind antigens on the surface of pathogens and flag them for destruction through complement activation, opsonin promotion of phagocytosis and elimination by immune effector cells. (awaiting scientific review)
  • Antibody diversity is produced by the mutation and recombination of approximately 300 different gene segments encoding the light- and heavy-chain variable domains, in precursor cells destined to become B cells. (awaiting scientific review)
  • IgG antibodies make up about 80 percent of all antibodies. (awaiting scientific review)
  • In inflammation, damaged cells release chemicals such as histamine that make blood vessels leak fluid into tissues, causing swelling, and that attract phagocytes which engulf germs and dead or damaged cells. (awaiting scientific review)
  • Neutrophils are the most abundant leukocytes of the immune system and are phagocytes that engulf and digest pathogens. (awaiting scientific review)
  • After a first (primary) response, antibody-producing memory cells remain, so if the body meets the same pathogen again the antibody response is much faster and more effective. (awaiting scientific review)
  • Vaccines work by causing acquired immunity without the person needing to have the infection first. (awaiting scientific review)
  • Vaccines contain weakened or inactive parts of a pathogen (antigens), weakened or reconstituted whole viruses or bacteria, or a DNA or RNA blueprint for producing an antigen, and they prompt an immune response without causing the disease in the person vaccinated. (awaiting scientific review)
  • When many people in a community are vaccinated, a pathogen has difficulty circulating, which also protects people who cannot be vaccinated (herd immunity), although no single vaccine provides 100% protection. (awaiting scientific review)
  • Passive immunity comes from antibodies made in another body: infants are born with antibodies transferred across the placenta from their mother, and these disappear between 6 and 12 months of age. (awaiting scientific review)
  • Because antibodies bind antigens so specifically, laboratory methods such as radioimmunoassays use the antigen–antibody interaction to detect and measure substances. (awaiting scientific review)
  • After decades of polio vaccination through routine immunisation and mass campaigns, the African continent was certified free of wild poliovirus in August 2020. (awaiting scientific review)
  • Immune system disorders include responses directed against the body's own tissue (autoimmune disorders), a lacking response (immunodeficiency), and allergies, which are immune responses to substances most people's bodies perceive as harmless. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.