Divergent selection of chickens for antibody production to sheep erythrocytes: age effect in parental lines and their crosses.

Age dependency of antibody response to sheep red blood cell (SRBC) antigen was measured in lines of chickens divergently selected for this trait and in reciprocal crosses between them. At 7 days of age, there were differences among populations for frequency of responders to SRBC antigen. This qualitative pattern persisted in the quantitative context of the antibody titers of those who responded, demonstrating genetic differences in both the event and subsequent levels of antibody. Although chickens from the high line had significantly higher titers than those from the low line and cross populations, all reached serological maturity by 14 days of age. From this age, high-line chickens had higher bursa and spleen weights and lower thymus weights relative to body weight than those from the low line: relative to body weight, spleen and bursa weights increased at a faster rate through 19 and 25 days of age, respectively, and then plateaued. In contrast, there was a progressive increase in thymus weight relative to body weight through 40 days of age.