The Fc-recognizing, collagen-like C1q molecule is a putative type II membrane protein of macrophages.

The Fc-recognizing, collagen-like C1q molecule, a subcomponent of the first component of complement, C1, is present on the cell surface of guinea pig peritoneal macrophages and human peritoneal and monocyte-derived macrophages. On closer examination, C1q appears to be a membrane protein (membrane C1q) of macrophages since it is i) anchored into the membrane throughout the biosynthetic pathway, ii) tightly and irreversibly bound to the cell surface and iii) only liberated if the intact membrane structure is disrupted by a detergent or repeated freeze/thawing. Additionally, the amino acid sequence of the A chain of human C1q displays properties that are characteristic for integral type II membrane proteins. The membrane C1q of guinea pig macrophages has a "lighter" B chain than serum C1q. Under physiological conditions in culture guinea pig macrophages release membrane C1q thereby converting it into the serum form. Moreover, membrane C1q appears to be involved in various cellular events such as binding of Fc, polyanions, lipid A and gram-negative bacteria to macrophages.