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The immune system is a network of cells, tissues and organs that fights
infections and other diseases. When bacteria or viruses invade, they attack
and multiply — that is an infection, and the infection causes the illness. The
immune system fights the germs off.
The parts
| Part | Job |
|---|---|
| Skin | Keeps germs out in the first place |
| Mucous membranes — the moist linings of some organs and cavities | Make mucus and other substances that trap and fight germs |
| White blood cells | Fight germs |
| The lymph system — thymus, spleen, tonsils, lymph nodes, lymph vessels, bone marrow | Produce, store and carry white blood cells |
How it works, and the part that matters most
The system defends against anything it reads as harmful or foreign. Those
things are called antigens: germs, chemicals, toxins, and also **your own
cells when they are damaged** — by cancer, or by sunburn.
When it recognises an antigen it attacks: that is an immune response. Part of
the response is making antibodies, proteins that attack, weaken and
destroy the antigen, alongside other cells built for the same job.
Then comes the important bit. **Afterwards, the immune system remembers the
antigen.** Meet it again and it recognises it, sends the right antibodies
quickly, and in most cases you do not get ill. That is immunity — and it is
the mechanism every vaccine is aimed at.
Three kinds of immunity
| Where it comes from | How long it lasts | |
|---|---|---|
| Innate | What you are born with — the skin, mucous membranes, and some cells and chemicals that attack foreign substances. Your first line of defence | Life |
| Active (adaptive) | Being infected, or being vaccinated | Usually long-lasting; for many diseases, your whole life |
| Passive | Receiving antibodies rather than making them — a newborn gets them from its mother, and blood products can carry them | Immediate protection, but only a few weeks or months |
Passive immunity is the odd one: it is the only kind that works instantly, and
the only kind that expires quickly, because nothing was learned. No memory was
written.
What goes wrong
- A response with no real threat — allergies, asthma, and autoimmune
diseases, where the immune system attacks healthy cells by mistake. - A system that does not work properly — immunodeficiency diseases,
often genetic, where you get ill more often and infections last longer, get
more serious and are harder to treat. - A system that is attacked directly — HIV destroys white blood cells,
and untreated it leads to AIDS, where the immune system is badly damaged
and severe illnesses accumulate.
Source: MedlinePlus, U.S. National Library of Medicine.
Licence: CC0 1.0 (public domain) · Adapted from medlineplus.gov
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