The ontogeny of the humoral immune response to E. coli vaccine and to sheep red blood cells in young chicks.

The ontogeny of the humoral immune response to sheep red blood cells (SRBC), a non-infective antigen, and to killed Escherichia coli (E. coli), a disease-related antigen, was compared in young chicks of three poultry populations: a White Leghorn layer strain and two strains of White Rock broiler breeders. SRBC were injected intravenously into randomly selected groups of chicks every 3 days from hatch until 42 days. In separate groups of chicks, E. coli (1 x 10(8) heat-inactivated organisms) were injected intravenously over the same age range. A clear difference was found in the response pattern to the two antigens with age. The response to SRBC developed rapidly: at 6 days 'of age most chicks showed measurable antibody titres. The response to E. coli developed more slowly: at 6 days of age most of the chicks in the three strains tested did not show measurable antibody titres. The age response pattern of the two White Rock strains tended to be similar to one another and to differ from the response of the White Leghorn strain. Considerable variation in antibody titre was found within each group of chicks. These results indicate that differences are present in the rate of development of the immune response to different antigens and suggest that genetic differences in the rate of development of the immune response to a given antigen may be present both between and within poultry populations. Thus, increasing the rate of development of the immune response in poultry populations may be a feasible breeding objective.