The C1q-R participates in immunoregulation and signal transduction.

The receptor for human C1q which recognizes the "collagen like" tail of its ligand is expressed on a wide range of cells including those which are not involved in the immune response. Nevertheless, the interaction of C1q with its receptor triggers a variety of cellular responses and these responses are suspected to involve certain signalling pathways. The few examples cited in this report provide ample evidence to support the long held notion that the cellular responses induced by C1q-C1q-R interaction involve signal transduction pathways. Platelets, for instance, can be activated by aggregated C1q. This activation, which is accompanied by a significant increase in the release of inositol-1,4,5-triphosphate (IP3) results in the expression of alpha 11b/beta 3 integrins (GPIIb/IIIa) and procoagulant activity and may contribute to thrombotic and inflammatory reactions. The ability of C1q to induce proliferative or anti-proliferative responses depend upon the nature of the C1q or the state of the cell expressing the C1q-R. In any event, even these responses most likely involve activation of transmembrane signal transduction pathway(s) which are yet to be elucidated.