High levels of natural antibodies (NAb) binding the -Gal residue (Gal1-3Galß1-4GlcNAc) are found in poultry (and humans), which is probably reflected by high levels of natural agglutinating antibodies (Ab) to rabbit red blood cells (RRBC) in plasma from chickens (and humans). Recently, it was shown that -Gal conjugation of proteins induced higher antiprotein Ab responses in -Gal knockout mice, suggesting immune-enhancing features of preexisting Ab binding carbohydrate-protein conjugates. We challenged chickens s.c. with either -Gal-conjugated human serum albumin (HuSA), ß-Gal-conjugated HuSA, or unconjugated ("native") HuSA, respectively, and measured primary and secondary Ab responses to HuSA, including isotype IgM and IgG responses, and cellular immune responses in vitro (lymphoproliferation) to HuSA or concanavalin A. -Gal conjugation, but not ß-Gal conjugation, of HuSA resulted in significantly decreased primary and secondary Ab responses to HuSA, especially IgG isotype responses, as compared with Ab responses to native HuSA. Lymphoproliferation in vitro was also decreased, although not significantly, in birds challenged with -Gal-conjugated HuSA. High levels of agglutinating Ab levels to RRBC and NAb binding porcine thyroglobulin were detected in all birds, as was true for (natural) Ab levels binding -Gal-conjugated HuSA before immunization, whereas low levels of preexisting (natural) antibodies directed to native HuSA were present in plasma before immunization. Levels of RRBC agglutinins and Ab binding thyroglobulin were not affected by immunization with HuSA, -Gal-conjugated HuSA, or ß-Gal-conjugated HuSA. Our data confirm the presence of high levels of (preexisting) NAb in the plasma of chickens directed to the -Gal residue. The decreased responsiveness to -Gal-bearing antigens in the current study shows that, in addition to immune-enhancing features, NAb may also have suppressive effects on specific immune responses, which substantiates the regulatory role of innate immunity (NAb) in mounting specific immune responses.