Pigs were bred for 8 generations for high (H), low (L) and control (C) immune responsiveness using estimated breeding values (EBV) of antibody (Ab) and cell-mediated immune responses (CMIR). Response to selection was determined by differences in least squares means and average EBV between the H and L lines. Lines diverged steadily between generations 1 to 3 (G1-G3); however, there was little or no response to selection following G4. Antibody responses to various antigens were significantly higher in the H line. Following infection with Mycoplasma hyorhinis in G4 and G8 H line pigs had significantly less peritonitis and pleuritis, but had more severe arthritis. Increased arthritis may be due to the formation of immune complexes or more severe inflammatory responses based on antigen-specific CMIR From GO to G7 H line pigs had higher rate of gain compared to L or C. If enhanced rate of gain is due to selection for H immune response and not due to founder, or other effects, then increased productivity may be the greatest benefit of this selection.

B. A Mallard, B. N Wilkie, B. W Kennedy, John P Gibson, M. Quinton

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