C1q attachment to antibodies
Complement factor c1q binds to the constant Fc portion of an antibody (Duncan &amp; Winter, 1988). Indeed, the constant (Fc) tail of the antibody is where other proteins involved in immune responses typically bind, most notably the Fc receptors (e.g., Clynes et al., 2000) found on immune cells such as B lymphocytes. Fc receptors mediate the recruitment of immune cells to a site of infection. The primary role of the Fab region is antigen binding as it contains the highly variable antigen binding domain. See the following figure (from the site of Invivogen) for the Fab and Fc region showing the constant and variable regions of antibodies:




  Figure legend from Invivogen): The Fc region mediates effector functions, such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). In ADCC, the Fc region of an antibody binds to Fc receptors (FcγRs) on the surface of immune effector cells such as natural killers and macrophages, leading to the phagocytosis or lysis of the targeted cells. In CDC, the antibodies kill the targeted cells by triggering the complement cascade at the cell surface. I


The constant regions of antibodies are located on the heavy chain (CH1,2,3) and the light chain (CL). These constant regions are potential binding sites for other immune molecules as they are constant between antibodies, irrespective of the variable region in Fab. Most of the constant regions are found on the Fc region explaining its major role in binding other molecules involved in the immune response, including complement factor c1q. However, the constant regions in Fab (CL) may also provide binding sites for other proteins and indeed some bacterial compounds are known to bind to this region (Bouvet, 1994). However, I have not found evidence of c1q binding the Fab region.