Disease resistance and immune response genes in cattle: strategies for their detection and evidence of their existence.

The possibility of breeding or genetically engineering cattle for resistance to disease has tremendous potential for increasing the efficiency of milk and meat production. In cattle and other species, major genes that control humoral and cellular immune responses to a variety of antigens have been mapped to a chromosomal region known as the major histocompatibility complex. However, resistance or susceptibility to viral, bacterial, and parasitic diseases in noninbred species is often a complex phenotype, with age, stress, and physiologic status all being important factors in the outcome of infection. This paper reviews the function of major histocompatibility complex gene products and the relationship between polymorphism of these genes and infectious diseases. A discussion of strategies for detecting immune response genes and disease associations is presented, with particular reference to the problems and advantages of working with cattle. The present knowledge of the bovine major histocompatibility complex and its relationship to immune responsiveness and disease resistance are also reviewed, with special consideration given to enzootic bovine leukosis because of the significant relationship between alleles of the bovine lymphocyte antigen system and resistance or susceptibility to subclinical progression of bovine leukemia virus infection. Finally, potential applications of this research to genetic improvement and animal health are considered.