Primary and Secondary Immune Response

The primary immune response happens the first time the body meets a particular antigen. It is slow: there is a delay of several days while clonal selection takes place, because only a few lymphocytes have complementary receptors and they must be activated and divide to form plasma cells. Antibody concentration in the blood rises slowly, peaking after about 10 to 14 days. During this delay the pathogen multiplies, so the person usually develops symptoms. The primary response leaves behind memory cells (B memory and T memory cells).The secondary immune response happens if the same antigen enters the body again, even years later. Because there are now many memory cells with complementary receptors, the response is:
• faster: antibodies appear within a day or two
• stronger: memory B cells divide rapidly into plasma cells, producing a much higher antibody concentration (often 10 to 100 times higher)
• longer lasting: antibody levels stay high for longer.The pathogen is usually destroyed before it can cause symptoms, so the person is immune.Worked example: the peak antibody concentration is 20 arbitrary units in the primary response and 900 arbitrary units in the secondary response. The secondary peak is 900 ÷ 20 = 45 times higher.Memory is specific. If a pathogen's antigens change, as in antigenic drift of influenza, the memory cells no longer match and a new primary response is needed.