CR1 (Complement Receptor 1, CD35) is a membrane receptor for C3b and C4b expressed on erythrocytes, leukocytes and podocytes. It plays an important role in removal of immune complexes and pathogens coated with C3b and C4b. It also regulates the complement cascade activation by preventing formation of classical and alternative pathway convertases and by acting as a cofactor for factor 1 mediated cleavage of C3b to iC3b, C3c and C3dg. CR1 is a polymorphic molecule differing in molecular weight and the level of the CR1 expression on erythrocytes. It takes part in pathogenesis and development of various autoimmune and infectious diseases. The difference in expression of CR1 seems to correlate directly with the development of systemic lupus erythematosus (SLE) and severe form of malaria. The therapeutical potential of soluble CR1 (sCR1) is at present the subject of many investigations.