Outcomes such as health performance, growth performance, and feed efficiency - outcomes that are clinically relevant and economically important to beef cattle producers and veterinarians involved in production medicine are the most clinically relevant and economically important outcomes by which to evaluate immune function in beef cattle populations. Although substitution indicators like serum antibody titers, lymphocyte proliferation assays and other laboratory assays may be indirectly related to the health and performance of the populations of interest, they are not directly correlated with the economically important outcomes essential to the financial health of the beef cattle businesses for which food animal veterinarians provide service. A number of factors or events impact immune function of cattle that are received and fed in North American feedlots. These include, but are not limited to, weaning, commingling, transportation, arrival processing surgical procedures, nutritional status and environmental conditions. The expression of immune function in terms of health performance, growth performance, and feed efficiency, during the feeding phase is affected by not only arrival conditions and procedures conducted at feedlot arrival or management during the feeding phase, but the background and history of management prior to feedlot arrival. These are the most economically important measures of immune function.