This is the part of the immune system with ‘memory’. It is involved positively in immunisation, and negatively in allergy and auto-immunity.

Once the acquired immune system has learned to recognise an enemy, after an initial infection or after a vaccination, it remembers the enemy’s characteristics.

On second exposure to the threat the memory cells recognise it, and generate an immune response involving highly specific weapons such as antibodies. That’s why it is very unusual to catch measles twice.

The acquired immune system is powerful and highly specific, but it is initially slow to respond and often insufficient to protect the host against the first onslaught of a virulent bacterium or virus.

It is only able to respond rapidly if you have already encountered the threat previously. And after an initial infection, small mutations in a virus may be enough to bypass the acquired immune system - one reason why it is only our second line of defence.

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