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Trace minerals exist in cells and tissues of the animal body in a variety of chemical combinations, and in characteristic concentrations, which vary with the trace mineral and tissue (McDowell, 1989, 1992; Underwood and Suttle, 1999). The concentrations of trace minerals must usually be maintained within quite narrow limits if the functional and structural integrity of the tissue is to be maintained and the growth, health, and productivity of the animal are to remain unimpaired (McDowell, 1989, 1992; Underwood and Suttle, 1999). Ingestion of diets that are deficient, imbalanced, or excessively high in trace minerals may induce changes in the form or concentration of the particular trace mineral in the body tissues and fluids, so that it falls below or rises above the tolerable limits. In such cases, biochemical lesions can develop, physiological functions may be adversely affected and structural disorders may arise, in ways which vary with the trace mineral, the degree and duration of the dietary deficiency or toxicity and the age, sex, or species of animal involved (McDowell, 1989, 1992; Underwood and Suttle, 1999). Certain homeostatic mechanisms in the body can be activated which delay or minimize the onset of such diet-induced changes. Ultimate prevention of the aforementioned changes require that the animal be supplied with a diet that is palatable and non-toxic and which contains the required minerals, vitamins, as well as other nutrients, in adequate amounts, proper proportions, and available forms (Underwood, 1971; Underwood and Suttle, 1999).
Trace mineral deficiencies, toxicities, and imbalances require the animal to metabolically compensate for the nutrient deviation. In doing so, certain metabolic diseases can manifest and immune function can be depressed, thus decreasing overall animal performance and health. The intent of this review is to discuss the function of trace minerals in health and immunity of livestock. For an in depth review of the effects of micronutrients on immunity see Galyean et al., (1999), and Spears (2000).