In the present study differences in the B cell compartment of two chicken lines selected for either high (H) or low (L) antibody response to sheep red blood cells (SRBC) are investigated. In non-immunized chicks, flow cytometry revealed generally more circulating Ig+ leukocytes in the H line, while in the L line slightly more CD4+ and in week 5, more CD8+ cells were found. In the L line spleen more CD8+ were found and in the H line spleen more CD4+ cells. In week 6, half of the chicks were immunized. Both lines were similarly affected by immunization. Immunization reduced the percentages of the circulating T cell subpopulations, while Ig+ cells were enhanced, compared with non-immunized chicks. Histological determinations with specific mAbs on spleens of young, non-immunized chicks, showed large dense T cell areas in the L line, while in the H line more and larger germinal centres were found. In the H line, also, more B cells were found in the peri-ellipsoid lymphoid sheaths (PELS). No line differences in mononuclear phagocytes were found other than associated with line differences in numbers of PELS and germinal centres. After immunization with TNP-BSA, both higher numbers of TNP-specific antibody producing cells and higher levels of circulating antibody were found in the H line. Moreover, more TNP-specific plasma cells were found in non TNP-immunized H line chicks, than in the L line chicks. The H line had also higher ELISA-titers to KLH 5 days after immunization with KLH. Therefore it was concluded that selection for antibody response has affected the B cell compartment. The H line has relatively more B cells and the splenic structure of the H line differs from the L line, in the H line probably resulting in a more optimal organization for antibody response to T cell dependent antigens.

Four experiments were conducted to determine possible differences in the in vitro concanavalin A (ConA) response between two lines selected either for high (H) or low (L) antibody response 5 d after intramuscular immunization with SRBC. In all four experiments, the cell proliferation after stimulation with ConA was higher, although not always significantly so, in the L line than in the H line, independently of dose of Con A and source of lymphocytes. It can be concluded that selection for anti-SRBC antibody response affected the cellular response in chickens. Previously reported results, in other chicken lines selected for humoral response to SRBC after intravenous immunization with SRBC, showed an opposite line difference in mitogen response. These opposite results point to the fact that comparable selection protocols for immunological variables do not necessarily have a comparable influence on the diverse components of the immune response.