Every pathogen has molecules on its surface called antigens. When a pathogen enters the body, this exposure to the pathogen triggers an immune response. White blood cells called lymphocytes recognise the antigens and produce antibodies. Each antibody has a shape that fits one specific antigen, so antibodies made against one pathogen do not work on a different pathogen. Antibodies bind to the antigens and help other white blood cells to destroy the pathogen.
The antigens also trigger the production of memory lymphocytes. Memory lymphocytes stay in the blood after the infection has gone. The first time the body meets a pathogen, the primary response is slow, so the person may become ill.
If the same pathogen enters the body again, the memory lymphocytes recognise its antigens. They quickly make large numbers of the correct antibodies. This secondary response is faster and bigger than the primary response, so the pathogen is destroyed before the person becomes ill.
