Discussion
Several differences between the F and RBC2 lines were observed in the immune system that might explain the line difference in disease resistance. The humoral responses seem to be similar between the lines but the cellular immune response was negatively affected in the heavy lines. Among the differences observed between the F and RBC2 lines were: 1) a higher response to the mitogen Con A in the RBC2 line than in the F line; 2) serum IgM concentrations were higher in the F line than in the RBC2 line; 3) CD8 molecules in the F line were more polymorphic than those in the RBC2 lines; 4) the F line had a larger CD4+CD8- T cell subpopulation than the RBC2 line; 5) phagocytosis, as measured by carbon clearance, was greater in the RBC2 line than in the F line; and 6) the F line had greater spleen weight and ratio of spleen to bursa of Fabricius weight but had smaller bursa of Fabricius weights than the RBC2 line; and 7) the F line had a higher antibody response to the SRBC antigen than the RBC2 line. The F and RBC2 lines did not differ in response to the mitogen PHA-M, serum IgG concentrations, antibody responses to B. aviam, Newcastle disease virus, and P. multocida.
The difference in disease resistance between the F and RBC2 lines was probably the result of many of the observed changes in the immune system. Many of the measurements are too expensive and time consuming to be applied to large populations in an effort to genetically improve disease resistance in large-bodied strains. One of the possible measures that might be used on large populations is the response to Con A mitogen when measured on whole blood. Studies are underway to determine if response to Con A could be used as a selection tool to improve disease resistance while not compromising growth rate.