Adv Nephrol Necker Hosp. 1989;18:249-69. Unique Identifier : AIDSLINE
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).
Antigen-Antibody Complex/METABOLISM Complement 3b/IMMUNOLOGY
Erythrocytes/PHYSIOLOGY Gene Expression Regulation Human Immunity,
Cellular Kidney Glomerulus/ANALYSIS/PHYSIOLOGY Phagocytosis
Receptors, Complement/ANALYSIS/IMMUNOLOGY/*PHYSIOLOGY JOURNAL ARTICLE
REVIEW REVIEW, TUTORIAL