Hemagglutination-enhancing factor (HEF) is a chicken serum macroglobulin first described on the basis of the ability of normal chicken serum to cause pronounced aggregation of allogeneic erythrocytes (RBC) sensitized with non-agglutinating levels of alloantisera. Subsequent work showed that HEF was distinct from the first component of complement, but the identity of HEF and its ligand(s) have not been defined. We found that chicken serum agglutinated homologous RBC and sheep RBC lightly sensitized with specific IgY but not sheep RBC sensitized with rabbit IgG. Thus, the sole ligand of HEF appears to be antigen-bound IgY. HEF activity was found to be heat resistant (56o for 30 min.), unaffected by chelation with 10mM EDTA, and was not inhibited by monosaccharide ligands of C-type lectins. An isolate was obtained from chicken serum following salt precipitation, gel filtration and ion exchange chromatography that contained potent HEF activity, and consisted almost exclusively of IgM. These findings suggest that HEF is a naturally occurring IgM antibody that has the capacity to interact with antigen-bound IgY.

Supported in part by the Ralph & Louise Bricker Endowed Fund