Immune System

What this part of the map covers

The immune system is a complex network of cells, tissues and organs that work together to help the body fight infections and other diseases.^[1] A pathogen is a bacterium, virus, parasite or fungus that can cause disease.^[2] When germs such as bacteria or viruses get into the body and multiply, that is an infection, and the infection causes the illness.^[1]

The first line of defence

Innate immunity is the protection people are born with, and it is the body’s first line of defence.^[1] It includes the skin, which can help keep germs out, and mucous membranes, the moist linings of some organs and body cavities, whose mucus can trap and fight germs.^[1] The WHO adds cilia, microscopic hairs that sweep debris away from the lungs, to these physical barriers.^[2] Innate immunity also includes some cells and chemicals that can attack foreign substances.^[1]

White blood cells and the lymph system

White blood cells fight germs.^[1] Organs and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes and bone marrow, produce, store and carry them.^[1]

Antigens and antibodies

An antigen is anything the immune system treats as harmful or foreign: germs, chemicals or toxins, or even cells damaged by things like cancer or sunburn.^[1] Recognising an antigen sets off an immune response, and part of that response is making antibodies, proteins that attack, weaken and destroy antigens.^{[1, 3]}

The WHO’s European office explains that white blood cells called B-lymphocytes make antibodies shaped to attach to one particular antigen.^[5] Once attached, they make it much easier for other parts of the immune system, particularly cells called phagocytes, to recognise and destroy the invader.^[5] The body holds thousands of different antibodies, and antibodies against one pathogen generally do not protect against another unless the two are very similar.^[2]

Memory: why a second encounter is different

The first time the body meets an antigen, it takes time to produce the matching antibodies.^[2] During that first response it also makes memory cells, which stay alive after the pathogen is defeated.^[2] The CDC (US) calls this lasting protection immunologic memory: memory B-cells circulate in the blood and live in the bone marrow for many years, and on a new exposure they multiply and make antibody rapidly.^[7] In most cases, this quick response means the person does not get sick, and that protection against a particular disease is called immunity.^[1]

The CDC (US) notes that age, nutrition, genetics and other illnesses can affect how well the immune system responds.^[7]

Kinds of immunity

• Active immunity (also called adaptive immunity) is made by a person’s own immune system after an infection or a vaccination.^{[1, 7]} It usually lasts a long time, and for many diseases it can last a lifetime.^[1]
• Passive immunity comes from receiving antibodies made by someone else.^{[1, 7]} It protects at once, but the antibodies break down over weeks to months.^[7]

How vaccines work

The CDC (US) explains that vaccines imitate an infection to engage the body’s natural defences, and that the active ingredient in every vaccine is an antigen.^[6] The NHS (UK) says most vaccines contain a small amount of a bacterium, virus or toxin that has first been weakened or destroyed in a laboratory.^[8] According to the WHO, newer vaccines carry the blueprint for making an antigen (DNA or RNA) instead of the antigen itself.^[2] The WHO says the vaccine does not cause the disease but still prompts a first-time immune response.^{[2, 3]}

The WHO notes that some vaccines are given as several doses, weeks or months apart, to allow long-lived antibodies and memory cells to develop.^[2] It says that protection then typically lasts years, decades or even a lifetime.^[3] The CDC (US) adds that immunity can take weeks to build, and that vaccinated people can sometimes still be infected, though they are far less likely to die or become seriously ill.^[6]

Herd immunity

The WHO explains that when many people in a community are vaccinated, a pathogen struggles to spread because most people it meets are immune, which also lowers the risk for those who cannot be protected by vaccines, such as very young babies or people who are seriously ill; this is herd immunity.^{[2, 3]} It also stresses the limits: no single vaccine gives 100% protection, and herd immunity does not fully protect people who cannot safely be vaccinated.^[2]

A short history

The WHO says that in the early 1900s polio paralysed hundreds of thousands of people every year.^[2] Its explainer reports that the African continent was certified free of wild poliovirus in August 2020, leaving Pakistan and Afghanistan as the places where polio had not yet been eradicated at the time of writing.^[2] On its undated topic page, the WHO states that immunization currently prevents 3.5 to 5 million deaths a year from diseases such as diphtheria, tetanus, whooping cough, influenza and measles.^[4]

When the immune system goes wrong

• Sometimes the immune system responds when there is no real threat, which can lead to allergies and asthma.^[1]
• In an autoimmune disease, the immune system attacks the body’s own healthy cells by mistake.^[1]
• In an immunodeficiency disease, infections happen more often and may last longer; these conditions are often genetic.^[1]
• HIV is a virus that damages the immune system by destroying white blood cells, and if it is not treated it can lead to AIDS.^[1]

Common misconceptions

“Several vaccines at once overwhelm the immune system.” The WHO says multiple doses do not overwhelm it, because vaccine antigens are a small fraction of what the body meets naturally every day.^[3]

“Being immune means you can never catch the disease.” The CDC (US) explains that immunity can be partial and is never perfect.^[6]

What this page does not cover

This is an overview. It does not describe the many kinds of white blood cell, T-cells, inflammation or fever, the causes and treatment of allergies and autoimmune diseases, or how vaccine safety is monitored. It gives no health advice.

Going further

See Microbes for the germs the immune system fights, Cells for the building blocks it is made of, Genetics for how traits are inherited, Medicine for the wider world of health care, and Body & Health for the rest of this region.