The immune system is complex, and the costs of an immune response are not yet widely understood, according to Dr. Kirk Klasing, a poultry scientist with the University of California-Davis, in a presentation at the 2017 Joint Annual Meeting of the American Dairy Science Assn. and American Society of Animal Science in Salt Lake City, Utah.

Klasing said immune response costs can be determined with direct estimates -- resources used by the immune system -- and indirect estimates, which are the trade-offs from the immune response. These rely on different sets of assumptions to get to a similar ending point.

In his abstract (185), Klasing reviewed the generalized immune response, noting that innate immune cells respond quickly to a potential pathogen due to the presence of a common set of receptors on all phagocytic cells that recognize broad categories of pathogens. Thus, a very large number of cells can recognize invading microbes and respond to them quickly.

A consequence of this is pathogen clearance, usually by phagocytosis, followed by the release of inflammatory cytokines and chemokines that amplify the local infiltration of additional inflammatory cells and activate them, he said.

If the challenge is large or if it is accompanied by damage to host tissue, cytokines are released in sufficient amounts that they have endocrine-like effects throughout the body, Klasing explained, noting that this cytokine storm induces metabolic changes, including increased protein degradation and insulin resistance in skeletal muscle, which diverts nutrients from muscle and other tissues so that they become available for the increased demands of leukocytes and for the production of protective proteins.

Importantly, the liver transitions from maintaining homeostasis and supporting the nutritional demands of growth or reproduction to the production of protective proteins such as complement, mannan-binding protein and C-reactive protein that aid in the detection and neutralization of pathogens.

Klasing detailed a study of the costs of a systemic inflammatory response in chickens to salmonella that examined the amount of nutrients in six different leukocyte types in five different tissues (blood, spleen, bursa, thymus, bone marrow) and 12 protective proteins (acute phase proteins and immunoglobulins). The study found that the amount of essential amino acids in the protective proteins greatly exceed that in the cellular component of the immune system during both a normal and an inflammatory state.

The ideal balance of amino acids for the acute phase of an inflammatory response differs greatly from that needed for growth, and there is a critical need for additional cysteine and threonine.

Ongoing research indicates that higher metabolic rate, decreased intake of food, a mismatch between the nutrient balance needed for the inflammatory response relative to that in body tissues and less efficient digestion that accompany a robust inflammatory response are, together, even more costly than the direct use of nutrients by inflammatory cells and the liver, Klasing reported.

Together, these costs result in decreased productivity that cannot be completely reversed by supplying additional nutrients.

He added that the direct estimate approach is lining up with the indirect approach, meaning that researchers are getting better at estimating the nutritional demands of an immune response similar to how dairy nutritionists account for every nutrient required in milk production.

Klasing's presentation was part of the American Registry of Professional Animal Scientists' symposium "Understanding Inflammation & Inflammatory Biomarkers to Improve Animal Performance."