Erythrocyte-antibody (EA) complexes made with chicken antibody (IgY) and sheep erythrocytes (SE) were used to detect antigen-antibody complex-binding cells in the chicken. Nonadherent mononuclear cells, adherent mononuclear cells, and heterophils were found to have a subpopulation of EA complex-binding cells. The binding of the EA complexes was dependent on the Fc portion of IgY since F(ab′)2 anti SE-SE complexes did not form rosettes with chicken mononuclear cells. Cells binding EA complexes were therefore referred to as Fc+ cells.

The percentage of Fc+ cells in the spleen, bursa, and bone marrow varied considerably in birds of different ages. No Fc+ cells were detected in the thymus until birds were 8 weeks old. No significant difference was seen in the percentage of Fc+ cells detected in spleens of bursectomized agammaglobulinemic birds and normal birds the same age. Over 80% of the Fc+ cells in a normal adult spleen stained with a nonspecific esterase stain and less than 5% carried a thymic cell antigen. Most (95%) of the Fc+ cells from bursectomized birds stained with a nonspecific esterase stain.

Rabbit anti-SE (IgG)-SE complexes did not bind to chicken mononuclear cells and the converse, chicken antibody (IgY)-SE complexes did not bind to mouse or rabbit mononuclear cells. A population of chicken mononuclear spleen cells was also identified which had a receptor for activated C in the form of erythrocyte-antibody (IgM)-C (EAC) complex. As with Fc binding, the binding of components of activated C was also homocytotrophic.

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