Figure 2.

A two-dimensional view of the adaptive (lymphocyte) immune response. Foreign antigen triggers the exponential proliferation of lymphocytes, which then differentiate into helper and effector cells. Regulatory mechanisms kick in at the peak of the response, the most conspicuous of which is the death of the majority of the lymphocytes by apoptosis. The few that survive become memory precursors and later memory cells. Lymphocyte death is necessary to prevent unwanted immunopathology.