The human C3b receptor (CR1).

The human complement system is comprised of 19 plasma components and regulatory proteins and of at least 9 distinct cellular receptors for these proteins or their activation fragments. The important role of complement in host defense against infection is related to its capacity to opsonize microorganisms, lyze target cells, and induce the release of inflammatory mediators from leukocytes. Complement participates in the processing and clearance of immune complexes and in regulation of the immune response. Most of the biologic effects derived from complement activation depend on ligand-receptor interactions between complement proteins or their cleavage fragments and specific receptors on cells. Two types of ligands are generated during complement activation: soluble low-molecular-weight ligands, such as the anaphylatoxins C3a and C5a, and so-called bifunctional ligands that attach both to the target of complement activation (opsonins) and to the appropriate receptor on effector cells. The most abundant complement protein in plasma is C3. Activation of the classic and alternative complement pathways generates C3 convertases that cleave C3 into an anaphylatoxic fragment, C3a, and a major fragment, C3b, which is capable of forming a covalent linkage with the targets of complement activation. Surface-bound C3b is the preferential ligand for the C3b receptor, CR1 (CD 35), which is expressed on most peripheral blood cells. The receptor plays an important role in the processing of immune complexes, the phagocytosis of C3b-bearing microorganisms, and regulation of the immune response. The cellular expression of the molecule is decreased in patients with systemic lupus erythematosus (SLE) and in patients infected with the human immunodeficiency virus (HIV).