The primary lymphoid organs - thymus and fetal bone marrow - store, release and confer competence on the lymphocytes that populate the secondary organs and CTs, but do not participate directly in defence.
Lymphocytes migrate in the blood and lymphatic flows for:
The initial colonization of spleen, etc;
a constant vigilant patrol by recirculation around the body, as memory or naive cells;
the propagation of an active immune response, as activated cells.
The secondary lymphoid organs provide:
APCs/reticular cells and macrophages to activate lymphocytes;
Many lymphocytes to respond to a major antigenic challenge coming via the blood (spleen) or lymph (nodes);
Lymphocytes to propagate the immune response further, say, to recruit other nodes ;
A cleansing action by macrophages to remove undesirable materials from blood and lymph undesirable.
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