| The regulation of lymphocyte circulation depends on the expression of markers of location and activation by the cells and tissues involved
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The entry of antigen into the lymphoid tissues has several effects on lymphocyte traffic into and out of these tissues, especially the lymph nodes. These effects occur in four phases:
- within hours, increased blood flow leads to increased lymphocyte entry. In the case of some antigens this is also accompanied by decreased lymphocyte outflow;
- by 1-2 days, there is an increased outflow of small, antigen-nonspecific lymphocytes; this represents the antigen-specific nonmigratory phase. Expansion of primed antigen-reactive B cells leads to the formation of primary follicle, in which the antigen-reactive B cells are located, to become a secondary follicle later;
- at 4 days, activated lymphocytes that have undergone blast transformation (blast cells) emigrate from the lymph node and home in with increased affinity on the high endothelial venule (HEV) to reach site of their first antigen contact, using adhesion molecules and their respective receptors;
- in the final phase, blast cell output falls and memory cells are generated.
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