White Leghorn chickens from lines selected for four immune-response traits (IR lines) were serotyped for B system alloantigens characterizing the haplotypes and genotypes to examine the effect of divergent selection for multitrait immunocompetence on MHC haplotype and genotype frequencies. The selected lines were derived from the Ottawa Strain 7. The selection index included four immunocompetence traits: antibody production against Mycoplasma gallisepticum (MG) and Pasteurella multocida, inflammatory response to phytohemagglutinin, and reticuloendothelial carbon clearance. The four lines include two replicates of high and low multitrait-immunocompetence lines. After four cycles of selection, significant differences (P \u3c .05) in several B system haplotype frequencies were observed, both among IR lines and between the IR lines and the Ottawa Strain 7. The B2 haplotype frequency was greater in all IR lines than in the Ottawa Strain 7. The B21 frequency was less in both high lines than in the Ottawa Strain 7. In comparisons among lines, frequencies of B21 were greater in both replicates of the low lines and the B12 and B19 frequencies were significantly greater (P \u3c .05) in the high lines. A gene substitution model showed effects (P \u3c .10) of specific haplotypes on MG and on the index. The B2 haplotype had a positive effect associated with MG. Haplotype B21 was positively associated with the multitrait index. Haplotype B13 had a negative effect on both MG and the index. Significant differences (P \u3c .01) in genotype frequencies were also noted among the IR lines. Associations between specific MHC haplotypes or genotypes and immune-response traits may offer insight into MHC-mediated mechanisms of disease resistance
