Complement receptors CD21 and CD35 in humoral immunity.

The complement system is a family of proteins that is involved in both innate and adaptive immunity. Complement receptors CD21 and CD35, which recognize activated products of C3 and C4, are predominantly expressed on B cells and follicular dendritic cells (FDCs) in the mouse. In this review, we focus on the role of FDC-expressed CD21 and CD35 in humoral immunity. They are the principle receptors for uptake and retention of immune complexes. In their absence, memory B-cell survival is markedly impaired. This is likely because of the lack of antigen but could also reflect a role for complement C3d ligand. How antigen is transported to FDCs remains an open question. In recent unpublished work using multiphoton intravital imaging, we found that small protein antigens presented in the lymph drain rapidly into B-cell follicles and are taken up by FDCs in a complement-dependent manner.