The immune system

The immune system protects the body from bacteria, viruses, fungi and parasites, and it also removes damaged cells and some cancer cells. It is not a single organ but a network of barriers, cells and molecules spread throughout the body.

Immunologists divide it into two broad parts that work together: innate immunity, which responds quickly to anything that looks foreign, and adaptive immunity, which is slower to start but targets specific invaders and remembers them.

• Skin forms a tough physical wall that most microbes cannot cross.
• Mucus in the airways traps particles, and tiny hairs called cilia sweep it away.
• Stomach acid kills many microbes swallowed with food.
• Tears and saliva contain enzymes such as lysozyme that break down bacteria.
• Friendly bacteria on the skin and in the gut compete with harmful microbes.

When microbes get past the barriers, innate immune cells respond within minutes to hours. Phagocytes, such as neutrophils and macrophages, engulf and digest invaders. They recognise general molecular patterns found on many microbes but not on human cells.

Inflammation is part of this response. Blood vessels widen and become leakier, letting immune cells and fluid reach the site, which causes redness, heat, swelling and pain. Fever raises body temperature, which can slow the growth of some pathogens and speed up immune activity.

Adaptive immunity relies on white blood cells called lymphocytes. The body produces an enormous variety of them, each able to recognise a different molecular shape, called an antigen. When one meets its matching antigen, it multiplies rapidly into an army targeted at that invader.

B cells produce antibodies, Y-shaped proteins that latch onto specific antigens, neutralising viruses and marking microbes for destruction. T cells come in several types: helper T cells coordinate the response, and cytotoxic T cells kill infected body cells directly. This targeted response usually takes several days to build the first time.

After an infection is cleared, some B and T cells remain as memory cells. If the same pathogen returns, they respond much faster and stronger, often destroying it before symptoms appear. This is why people usually get diseases like measles only once.

Vaccines use this memory safely. They expose the immune system to a harmless form or piece of a pathogen, such as a weakened or inactivated virus, a protein or genetic instructions for making a protein, so the body builds memory without suffering the disease. In 1796 Edward Jenner showed that material from cowpox could protect against smallpox, and vaccination eventually led to smallpox being declared eradicated in 1980.

An immune system that overreacts can cause allergies, in which it attacks harmless substances like pollen or peanuts. Severe allergic reactions, called anaphylaxis, are medical emergencies. In autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis and multiple sclerosis, the immune system attacks the body's own tissues.

An immune system that underperforms leaves people vulnerable to infection. This can be caused by inherited conditions, infections such as HIV, which destroys helper T cells, or treatments such as chemotherapy. Researchers now also harness immunity against disease, for example with cancer immunotherapies that help T cells recognise and attack tumours.