The specific immune system

Health, disease and the development of medicines · Human defences and immunity · note 2 of 4

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The specific immune systemSpec 5.13

If a pathogen gets past the first defences, the specific immune system responds. It is called specific because the response is aimed at one particular pathogen.

Antigen: A molecule, usually on the surface of a pathogen, that the immune system recognises as foreign and that triggers an immune response.
Antibody: A protein made by white blood cells (lymphocytes) that binds to a specific antigen.
Memory lymphocyte: A white blood cell that stays in the body after an infection and can respond quickly if the same antigen returns.

The primary response

The body is exposed to a pathogen.
The antigens on the pathogen trigger an immune response.
Lymphocytes with antibodies that fit the antigen divide, and the response causes the production of antibodies.
The antibodies bind to the antigens on the pathogen and help to destroy it.
The antigens also trigger production of memory lymphocytes.

The secondary response

If the same pathogen enters the body again, the memory lymphocytes recognise its antigens. They respond faster and make more antibodies than the first time, so the pathogen is destroyed before it can make the person ill. The person is now immune to that pathogen.

Graph of antibody concentration in the blood against time, showing a small, slow primary response after the first exposure to an antigen and a much faster, higher secondary response after the second exposure to the same antigen. — Memory lymphocytes make the secondary response faster and bigger than the primary response.