Kinetics of primary antibody responses to sheep red blood cells in birds: a literature review and new data from great tits and European starlings

Measuring the primary antibody immune response to an injection with sheep red blood cells (SRBC) is a routine application in poultry science. Recently, this technique is also becoming an increasingly popular tool to investigate humoral immunocompetence in free-living birds in studies of ecological immunology and immunotoxicology. However, an extensive search of the literature revealed that many of these studies have been measuring the primary immune response to SRBC without verifying first when maximum levels of antibodies are reached in the bird species under study. In addition, most studies assessed antibody titres to SRBC approximately 6 days after their inoculation assuming a similar pattern as found in poultry. We tested this assumption of a uniform pattern of kinetics of the primary humoral immune response to SRBC across bird species and investigated it in detail in two important model species, namely the great tit Parus major and the European starling Sturnus vulgaris. In general, the pattern was found to be the same in both passerine species and strongly resembled the pattern observed in chickens. Maximum antibody levels (the plateau phase) were reached on, respectively, day 5 and day 6 (with day of inoculation = day 0) of the immune response in the great tit and the starling and lasted for 3 days in both species. We found no effects of age, sex or time spent in captivity. Furthermore, our findings suggest that antibody levels to SRBC should not be obligatory assessed during the plateau phase of maximum antibody levels although it still remains most accurate to do so or at least shortly before or after it. Overall, we conclude that antibody levels have been reliably assessed in most avian studies using this technique.