The Immune System

Your immune system is your body's defence force. Every day, your body is invaded by countless bacteria, viruses, fungi and other tiny threats that could make you sick. The immune system spots them, attacks them, and remembers them so it can defend you even better next time. It is made up of dozens of different kinds of white blood cells, plus chemicals called antibodies, all coordinated through organs like the spleen, thymus, lymph nodes and bone marrow. When the immune system goes wrong (either too weak or too strong), it can cause serious health problems.

  • White blood cell typesapprox. 20+Each with specialised jobs
  • White blood cells in bloodapprox. 1% of blood cellsFar fewer than red blood cells
  • Antibody types5 main classesIgA, IgD, IgE, IgG, IgM
  • Where immune cells formBone marrowInside larger bones
  • Where T cells matureThymusA small organ in the upper chest
  • Lymph nodes in bodyapprox. 500 to 600Filter germs out of lymph fluid

The two halves of the immune system

Your immune system has two main parts that work together.

  • Innate immunity: the first line of defence. Fast but non-specific. Includes physical barriers (skin, mucus, stomach acid), inflammation, and white blood cells called phagocytes that swallow up invaders. You are born with this system fully formed.
  • Adaptive immunity: the second line of defence. Slow to start (takes days) but very specific. Includes B cells (which make antibodies) and T cells (which kill infected cells). Learns from experience, so it fights better the second time it meets a germ.

White blood cells: the immune army

Your blood contains several types of white blood cells, each with a different job.

  • Neutrophils: the most numerous. Fast-acting cells that swallow and destroy bacteria. The pus that forms in an infected wound is mostly dead neutrophils.
  • Macrophages: bigger and slower. Eat up debris, dead cells and germs. Also help train other immune cells.
  • Lymphocytes: split into B cells (make antibodies), T cells (kill infected cells), and natural killer cells (kill cancer cells and virus-infected cells).
  • Eosinophils: attack parasites and play a role in allergies.
  • Basophils: release histamine, which causes the redness and swelling of allergies and inflammation.

How antibodies work

An antibody is a Y-shaped protein produced by B cells. Each antibody is custom-shaped to fit a particular feature on a particular germ. When antibodies find their target germ, they stick to it and either:

  • Neutralise it (stick to it so it cannot infect cells).
  • Tag it for destruction by other immune cells (macrophages eat antibody-tagged targets preferentially).
  • Trigger a chain reaction of other immune chemicals that destroy it.

Your body can produce billions of different antibodies, each one shaped for a different threat. Once you have made an antibody, your body keeps a memory of how to make it again, so next time you meet the same germ you can produce huge amounts of the right antibody within hours.

The immune system's memory

The most remarkable feature of the immune system is its memory. After you fight off a particular infection, your body keeps a stockpile of memory B cells and memory T cells specifically tuned for that germ. If you meet the same germ again, these memory cells multiply rapidly, your antibodies kick in within hours, and you may never even notice you were infected.

This is why you usually only catch measles, chickenpox or mumps once: after surviving the first infection, your immune system can defeat the same germ almost instantly forever. Many other diseases (like the common cold or flu) avoid this trick by mutating constantly into new forms that your memory cells cannot recognise.

How vaccines work

A vaccine is one of the most clever medical inventions ever. It works by giving your immune system a "preview" of a dangerous germ without giving you the actual disease. The vaccine contains either:

  • A weakened or killed version of the germ.
  • Just a piece of the germ (often a protein from its surface).
  • (For modern mRNA vaccines) instructions for your own cells to make a harmless piece of the germ.

Your immune system reacts as if you had the real infection: it produces antibodies and trains memory cells. When the real germ comes along later, your body is already prepared and can defeat it before you get sick. Vaccines have saved hundreds of millions of lives and have wiped out diseases like smallpox completely.

Fact Vaccines are one of the most successful health interventions in history. Smallpox, a disease that killed roughly 300 million people in the 20th century alone, was completely eradicated by a global vaccination programme. The last natural case was in 1977, and the disease has not existed in the wild since. Polio, which used to paralyse hundreds of thousands of children every year, is now down to just a handful of cases worldwide and is on track to be the second human disease ever wiped out by vaccination.

When the immune system goes wrong

The immune system is incredibly powerful, which means problems can arise when it does not work properly.

  • Allergies: the immune system over-reacts to harmless substances (pollen, peanuts, pet dander). Hay fever, asthma and food allergies are all caused this way.
  • Autoimmune diseases: the immune system attacks the body's own cells by mistake. Examples include type 1 diabetes (attacks insulin-producing cells), multiple sclerosis (attacks nerve cells) and rheumatoid arthritis (attacks joints).
  • Immunodeficiency: the immune system is too weak to defend properly. Can be caused by genetics, malnutrition, certain medications, or infections like HIV.
Did you know? Your spleen is the largest organ of your immune system. It is about the size of your fist and sits under your ribs on the left side. The spleen filters old red blood cells out of your blood, stores white blood cells and antibodies, and is a major site for the immune system to respond to bacteria in the blood. People can survive without a spleen (in case of injury), but they are slightly more vulnerable to certain bacterial infections for the rest of their lives.
Deeper dive: bacteria vs viruses, and why antibiotics do not help with colds

People often lump "germs" together, but bacteria and viruses are completely different kinds of thing, and the difference matters for treatment.

Bacteria are single-celled living organisms. They have their own cell walls, their own DNA and their own machinery for reproduction. Most bacteria are harmless or beneficial; only a small minority cause disease. Bacterial infections include strep throat, tuberculosis, urinary tract infections, food poisoning from Salmonella or E. coli, and most ear infections. Bacterial infections can be treated with antibiotics, which target specific features of bacterial cells (such as their cell walls or their ribosomes) without harming human cells.

Viruses are much smaller and not really alive in the usual sense. They are essentially tiny packages of genetic material (DNA or RNA) wrapped in protein. They cannot reproduce on their own; they have to invade a living cell, hijack its machinery and force the cell to produce more viruses. Viral infections include the common cold, flu, measles, chickenpox, COVID-19, HIV and many others. Antibiotics do not work on viruses, because viruses do not have the kind of cell features that antibiotics target. The only treatment for most viral infections is for your immune system to fight them off.

This is why it is dangerous to ask for antibiotics for every cough or cold. Taking antibiotics when you do not need them does nothing for the virus AND helps breed antibiotic-resistant bacteria, which is becoming one of the biggest health crises of our time. The World Health Organisation now warns that the rise of antibiotic-resistant "superbugs" could undo many of the medical advances of the last century if we do not use antibiotics wisely.

For the blood that the immune cells use to travel, see the circulatory system. For the body's outer barrier, see skin and the integumentary system.