Primary and secondary immune responses.

In a primary response, as shown in this graph, an initial exposure to an antigen produces modest levels of specific antibody over a period of weeks. In a secondary response, subsequent exposure to the same antigen results in greater antibody production (note that the scale of the y-axis on the graph is logarithmic) that occurs more rapidly and lasts longer than a primary response. The secondary response is specific for that antigen. Exposure at that time to other antigens for the first time produces the usual primary response.