Host humoral factors in natural resistance to Haemophilus somnus

Natural serum bactericidal activity against Haemophilus somnus in various age groups of cattle was investigated. The level of antibacterial reactivity correlated with the agglutinating titer and age of the given animal. This activity was complement dependent, but the concentration of bactericidal antibody appeared to be the limiting factor in sera which had reduced killing capacity. Immunized animals developed high serum concentrations of bactericidal activity. The results of this study indicated that bactericidal antibodies may be important in host defense from H somnus infection and that immunization can enhance this resistance by stimulating high serum concentrations of bactericidal activity.