The antigen brought by lymph is phagocytosed by the interdigitating DC in paracortex, and presented to TH cells. The initial activation of B cells also occurs in the paracortex. The activated B cells move into foci at the edge of the paracortex, and develop into plasma cells 3-4 days after antigenic stimulation. Subsequent antigenic stimulation due to the antigen trapped by the follicular DC, leads to the development of the secondary follicle, with a germinal center. Division of B cells takes place, followed by differentiation into plasma cells or apoptosis.
Plasma cells migrate to the medulla.
The increase in cell number results from proliferation within the lymph node, but also from the addition of blood lymphocytes which arrive into the node by passing between the endothelial cells lining the postcapillary venules. The marginal zone of PALS is rich in B cells organized in primary follicles. The activated B cells migrate to follicles. Splenectomy in children leads to increased incidence in bacterial sepsis. In adults, the effects are less severe, there may be increases in bacteremia.