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* If your white blood cells have made a particular antibody in the past they can

remember how to make that particular antibody.

* If you have an illness caused by a particular pathogen your white blood cells

produce the correct antibody and destroy the pathogen.

* If you are infected by exactly the same identical pathogen your white blood cells

will remember how to make the correct antibody, produce it quickly and

destroy the pathogen before it can make you ill.

* If the pathogen is different it will have different antigens on its surface and your

white blood cells will need to make a different antibody to destroy it. – You can

become ill while you are waiting for your white blood cells to make the correct

antibody.