B cells

B cells, like T cells, recirculate and their first encounter with antigen may be in the T cell zone of lymphoid tissue where they meet antigen in clusters with interdigitating cells and T cells. Antigen-reactive B cells then migrate to the follicles to form a germinal center where a second encounter with antigen, in the form of immune complexes on the surfaces of follicular dendritic cells (FDCs), takes place. Further development to antibody-forming or memory cells also requires contact with CD40 ligand (CD40L)-positive germinal center T cells. All of these processes require active locomotion by B cells. Like T cells, B cells respond much better to attractants once they are activated.

Resting (G0) B cells acquire locomotor activity during culture in vitro with T cell products, IL-4 and anti-CD40 (which mimics CD40L), and the motile cells are in the G, phase of growth. After a few hours of culture with these factors, the B cells are capable of locomotor and chemotactic responses to antiimmunoglobulin (anti-IgM and anti-IgD). Germinal center B cells die rapidly in culture unless they receive rescue signals, and anti-CD40 provides such a signal. Germinal center cells not only survive but become motile in culture in anti-CD40+IL-4. These cells, again, are attracted by anti-immunoglobulin and, since they have switched isotypes, they are capable of recognizing anti-IgA and anti-IgG as locomotor signals. This suggests that antigen may be a natural chemoattractant for B cells. It is possible, but has not been investigated, that germinal center cells migrate in antigen gradients towards FDCs. Immune complexes activate complement and it has been reported that germinal center B cells cultured with FDCs respond to C5a as a chemotactic factor. This could also form a gradient which would bring B cells into contact with FDCs.

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