cr1

CR1, short for complement receptor type 1, is a membrane glycoprotein primarily expressed on human erythrocytes but also found on B cells, neutrophils, monocytes, and certain other cell types. It is encoded by the CR1 gene and is also known by the cluster of differentiation designation CD35. CR1 plays a central role in the regulation of the complement system and in the clearance of immune complexes from the circulation.

Functionally, CR1 binds complement fragments C3b and C4b deposited on pathogens or immune complexes. This allows immune complexes circulating in the blood to be bound by CR1 on red blood cells and transported to the liver and spleen for phagocytic clearance. CR1 also serves as a cofactor for factor I-mediated proteolysis of C3b and C4b, helping to attenuate the amplification of the complement cascade and limit tissue injury.

Structurally, the extracellular portion of CR1 consists of four long homologous repeats (LHRs A–D), each built from multiple short consensus repeats (SCRs, also called CCP domains), totaling about 28 SCRs. The receptor is anchored in the membrane by a single transmembrane segment, with a cytoplasmic tail involved in intracellular signaling.

Genetic variation at the CR1 locus influences receptor density and isoform size, with several common variants (eg, CR1*1, CR1*2) and copy number differences affecting expression. These variations have been studied for associations with susceptibility to infections, autoimmune conditions, and Alzheimer’s disease risk in genome-wide analyses, though mechanisms remain under investigation. In malaria, RBC CR1 density can affect parasite interactions and disease progression.

Note: CR1 may have other meanings in different fields, but complement receptor type 1 is the most well-known biological use.