CR1 (CD35) and CR2 (CD21) are structurally related integral transmembrane 
glycoproteins that function as cellular receptors for human C3b and C3dg, 
respectively. The primary sequence of the most common structural allotype of CR1 
and that of CR2 have been established, and ligand binding on the molecules has 
been mapped. CR1 and CR2 genes are located in close vicinity in the RCA locus of 
chromosome 1. CR1 has a wide cellular/tissular distribution and mediates a 
variety of biologic functions, including the transport of C3-bearing immune 
complexes on erythrocytes, enhancement of phagocytosis, induction of IL-1 
secretion and enhancement of B-cell differentiation. Expression of CR2 is 
restricted to B lymphocytes and follicular dendritic cells. The receptor 
modulates B-cell growth. CR2 also serves as the receptor for EBV and determines 
the cellular tropism of the virus. This review discusses the molecular biology 
and functional characteristics of CR1 and CR2. It focuses on alterations of 
expression of the receptors in disease, with particular emphasis on the genetic 
and acquired factors that contribute to the defective expression of CR1 in 
patients with systemic lupus erythematosus.