Complement receptor type 2 (CR2/CD21) as a central link between innate and acquired immunity

The complement system is activated by many molecules, including several important proteins of the innate immune system, that results in the covalent linkage of C3 activation fragments to Ag and/or Ab. Receptors for C3 activation fragments, especially CD21, play central roles in linking the B lymphocyte adaptive immune system to the complement system. CD21 deficient mice demonstrate pronounced immune defects. These defects are apparent because CD21 plays an important role in the activation of B lymphocytes, FDC, and likely T lymphocytes in vivo. Manipulation of CD21 activities by either blocking the binding of ligands to CD21, or targeting Ags to this receptor in vaccination strategies, are currently being explored as options to regulate several human immune responses. The regulation of immune responses through these and other strategies will likely further define the pivotal role CD21 plays in connecting innate to acquired immunity.