The appearance and nature of opsonins for goat erythrocytes during the development of the chicken.

The appearance of opsonins to goat erythrocytes during embryonic development of the chick has been examined with respect to increase in total macrophage population and increasing incorporation of maternal antibody. The decline in opsonizing power of serum shortly after hatching, probably due to the decline of maternal antibody, is offset by the active production of opsonins. Transfer of increasing concentrations of adult immune sera to embryos resulted in proportionate increases in clearance rates, showing that immune antibody was an efficient opsonin . Treatment of normal and immune adult sera before introduction into the chick embryo's circulation indicated that some of the natural opsonin was heat-labile at 56° and that reductively cleaved IgM antibody was not opsonic. IgG and IgM sera of similar titre had similar opsonizing power.