Immune System and Defence Against Disease

What the immune system does

The immune system defends the body against pathogens, the microorganisms that cause disease, such as bacteria, viruses, fungi and parasites. It also removes damaged and abnormal cells. Unlike the heart or lungs, it is not one organ but a network of barriers, organs, cells and chemicals spread through the whole body. Its defences work in layers: barriers keep pathogens out, innate immunityresponds quickly to anything foreign, and adaptive immunity builds a targeted response and remembers it.

The immune organs

All white blood cells are made in the bone marrow, the soft tissue inside bones such as the femur and sternum. B cells mature there, while T cells move to the thymus in the chest, where they learn to ignore the body’s own cells. The lymphatic system collects fluid that leaks out of capillaries into tissues. This fluid, called lymph, flows through lymphatic vessels and passes through hundreds of lymph nodes, small filters packed with B and T cells, before returning to the blood near the heart. The spleen filters the blood in the same way, and the tonsils guard the throat. Swollen lymph nodes in the neck during a cold are a sign that lymphocytes there are multiplying.

First line: barriers

Most pathogens never get inside. Unbroken skin is a tough, dry barrier. The airways are lined withmucus that traps microbes and cilia that sweep it away. Stomach acidkills many swallowed microbes, and tears and saliva contain enzymes that break down bacteria. A cut or burn breaks this barrier, which is why the simulator introduces its pathogen through a wound.

Innate immunity

The innate response starts within minutes and is the same for every pathogen. Damaged cells release chemicals such as histamine, causing inflammation: nearby capillaries widen and become leakier, so the area becomes red, swollen, warm and sometimes painful. White blood cells called phagocytes(neutrophils and macrophages) squeeze out of the capillaries, move towards the pathogens andengulf them in a process called phagocytosis, destroying them with enzymes. Chemicals released during infection can also reset the brain’s thermostat, causing a fever. Innate immunity often clears small infections alone; if not, it holds the pathogen back while the adaptive response prepares.

Adaptive immunity

Every pathogen carries antigens, molecules that the immune system recognises as foreign. Phagocytes carry pieces of these antigens to the lymph nodes and present them to lymphocytes. Each B or T cell has receptors that fit just one antigen, so at first only a few cells can respond. When a matching cell is found it divides again and again, a process called clonal expansion, which is why this response takes several days the first time.

• B cells become plasma cells that release antibodies, Y-shaped proteins that stick to antigens. Antibodies clump pathogens together, neutralise toxins and mark pathogens for phagocytes.
• Helper T cells coordinate the response and activate B cells.
• Cytotoxic (killer) T cells destroy body cells that have been infected by viruses.

Immune memory and vaccines

After an infection, most of the expanded B and T cells die, but some remain as long-lived memory cells. If the same pathogen returns, they recognise it straight away. The secondary response starts sooner, produces far more antibodies and usually clears the pathogen before it can cause illness. This is what being immune means. Vaccines use this: they contain a harmless form of an antigen, such as an inactivated pathogen or a piece of its protein, so the body makes memory cells without having the disease. When many people are vaccinated, a pathogen struggles to spread, which also protects people who cannot be vaccinated; this is called herd immunity.

The immune system and health

Sometimes the immune system responds when it should not. In allergies it reacts to harmless substances such as pollen; in autoimmune diseases, such as type 1 diabetes, it attacks the body’s own cells. In immunodeficiency, for example in HIV infection, it is weakened and infections become more dangerous. Antibiotics kill bacteria but have no effect on viruses, and their overuse helps antibiotic resistance evolve.

Key terms

Pathogen

A microorganism that causes disease.

Antigen

A molecule recognised by the immune system as foreign.

Antibody

A Y-shaped protein made by plasma B cells that binds to a specific antigen.

Phagocytosis

The engulfing and digestion of a pathogen by a phagocyte.

Lymphocyte

A B or T cell, the white blood cells of adaptive immunity.

Common misconceptions

• “Antibodies stay high for life after an infection.” Antibody levels fall; it is memory cells that last.
• “Antibiotics cure colds.” Colds are caused by viruses, and antibiotics only work against bacteria.
• “A fever is always harmful.” A moderate fever is part of the body’s defence, although very high fevers need medical attention.

Key takeaways

• Barriers stop most pathogens; innate immunity responds quickly with inflammation and phagocytes.
• Adaptive immunity uses B cells, antibodies and T cells that target one specific antigen.
• White blood cells are made in the bone marrow; T cells mature in the thymus; lymph nodes and the spleen filter lymph and blood.
• Memory cells make the secondary response faster and larger, which is how vaccines work.

Model note: pathogen numbers, cell numbers and antibody levels are in relative units from a small set of equations tuned to show textbook-shaped curves. Real responses involve many more cell types and signals and vary greatly between pathogens and people. See our educational and medical disclaimer.