CR3 (CD11b, CD18): a phagocyte and NK cell membrane receptor with multiple ligand specificities and functions.

The C3 receptor CR3 is expressed on phagocytic cells, minor subsets of B and T cells, and natural killer (NK) cells. It has important functions both as an adhesion molecule and a membrane receptor mediating recognition of diverse ligands such as intercellular adhesion molecule-1 (ICAM-1) and fixed iC3b. The receptor is capable of undergoing an activation event that regulates both its specificity for various ligands and its ability to mediate phagocytosis or extracellular cytotoxicity. Certain bacteria express carbohydrates or lipopolysaccharides (LPS) that can bind to and activate CR3, allowing the receptor to assume its activated state. Soluble beta-glucan derived from the yeast Saccharomyces cerevisiae is a particularly potent stimulator of CR3, and produces an activated state of the receptor that permits neutrophil phagocytosis of iC3b-coated erythrocytes or NK, cell cytotoxicity of iC3b-coated tumour cells, that are normally resistant to NK cells.