Antibody response to Haemophilus somnus Fc receptor.

To characterize the bovine immune response to an Haemophilus somnus antigen known to be recognized by convalescent-phase serum, we studied isotypic antibody titers to the 270-kilodalton protein, which we had previously shown to be an immunoglobulin Fc receptor. With a modified immunodot procedure, an immune response was detected after experimental H. somnus abortion, experimental H. somnus pneumonia, or vaccination with commercial H. somnus vaccine, with the greatest titer found within the immunoglobulin G2 isotype. With protein A peroxidase conjugate, which detects primarily bovine immunoglobulin G2, we showed that cattle with H. Somnus disease could be distinguished from clinically normal carriers, culture-negative cattle, or cattle with disease due to Pasteurella haemolytica or P. multocida. Little cross-reactivity between the 270-kilodalton Fc receptor antigen and antigens from other gram-negative bovine pathogens was seen. Thus, this antigen may be a useful diagnostic antigen.