Molecular Structure and Expression of Anthropic, Ovine, and Murine Forms of Complement Receptor Type 2

Complement receptor type 2 (CR2), also called CD21, is a cell surface glycoprotein expressed on the surfaces of mature human B lymphocytes, follicular dendritic cells (FDCs), a few T cells, and some striated epithelia (1).Two splice variants of CR2 have been detected in human B cells.The extracellular region of the prototype "short" form (145 kDa) contains 15 tandem short consensus repeat (SCR) domains (1), while the "long" form of CR2, containing 16 SCR domains, has been detected at the transcript level (58).Human CR2 associates with other molecules on the B-cell surface to form two distinct complexes.The most commonly expressed form of CR2 consists of a noncovalently associated complex formed between CR2 and CD19/CD81/Leu13 (104), although CR2 can also form complexes with CR1 (CD35) (110).The two complement receptors that are expressed by B lymphocytes both bind fragments of C3 that become covalently attached to activators of complement, providing the B cell with a nonclonal means for interacting with antigen that has been identified by this innate immune system.Therefore, CR2 rather than CR1 was the more likely candidate for the receptor that communicated innate recognition of antigen to the B lymphocyte (29).