When antigens are first encountered, the primary immune response occurs causing an increase in the antibody titer. The antibody titer is the amount of antibodies in serum of the infected organism. There are no or undetectable levels of antibodies when the antigen first attacks the organism. Then the antibody titer gradually increases and then declines as the antigen is destroyed or neutralized.
When antigens are encountered for the second time, the secondary immune response occurs, causing memory cells to quickly transform into plasma cells that produce antibodies. The secondary immune response is also known as the anamnestic response or memory response. A memory cell is a B lymphocyte that was generated in the primary immune response but did not became an antibody-producing plasma cell at that time.
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