When an individual encounters the same antigen for second time, immune system responds very quickly due to the presence of memory cells. Secondary immune response is different from the primary response, both qualitatively and quantitatively.
The log phase in secondary response is very short or negligible. Since the population of memory B cells specific for the antigen is present, the antibody levels peak with in a short time.
Hence lag face is absent in secondary response. Raise of antibody concentration is about 100 to 1,000 fold higher than the primary response resulting a short log phase. Like the sharp raise in Abs, the plateau phase is also much extended in secondary response.
The decline in the Abs level is much gradual than the decrease in primary response. Generally the Abs produced in secondary response is IgG type.
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Since IgG type Abs have longer life span (about 25 days), their concentration declines very steadily even in the absence of new plasma cells production.
This results much extended plateau phase and gradual drop in the decline phase.
Cell mediated immune response also follows the same tendency, but cytotoxic cells are the effectors cells in CMI.
For example in mouse, rejection of skin graft takes place in about 10-14 days for the first transplant, but when skin tissue from the same source is grafted for the second time, it is rejected-in about 7-9 days due to the faster action of memory T cells.