The B and T-lymphocytes that are yet to encounter an antigen are called naive B and naive T-cells. The naive B-cells that encounter the antigen, proliferate and differentiate into two types of cells: the antibody-secreting plasma cells and the memory B-cells.
The plasma cells form the basis of primary immune response, which is the response mounted by the immune system to an antigen that the animal encounters for the first time. The primary response has a characteristic lag phase, during which naive B-cells proliferate and differentiate into plasma cells and memory cells.
Following this, serum antibody level increases logarithmically, reaches the peak at about day 14, remains at a plateaus for some time, then begins to drop off as the plasma cells begin to die. The memory cells remain in G0 phase, and have a much longer life than plasma cells; some memory cells persist for the life of the individual.
Therefore, when the animal encounters the same antigen a second time, the population of memory cells responds rapidly to begin antibody secretion. The antibody levels peak in about 7 days, and the level of antibody is about 100 to 1,000-fold higher than that in the primary response.
The immune response mounted by the animal to an antigen, which it encounters a second time is called secondary immune response. The population of memory B-cells specific for a given antigen is considerably larger than the population of corresponding naive B-cells; this accounts for some of the differences between primary and secondary immune responses (Fig. 41.4).
In a similar manner, the recognition of an antigen-MHC complex by a specific mature T-lymphocyte induces its proliferation and differentiation into TH cells and CTLs (the effector cells) and into memory cells.
The effector cells bring about the primary immune response, which is relatively slower; it takes about 10-14 days in mouse for rejection of a skin graft in the first instance. But when skin tissue from the same source is grafted the second time, it is rejected in about 7-9 days due to the faster action of memory T-cells.