Zinc performs a number of unique functions in immunology, which distinguish it from all other trace elements. This special role is based upon its properties as a catalyst of a larger number of enzyme-controlled metabolic processes. Zinc supports, it even enhances, humoral and cell-mediated immunity by facilitating proliferative reactions to stimulus by different mitogens. This is as a result of its acting, above all, on the cell as co-factor for 24 presently known, important enzymes, by exercising a biocatalytic influence and regulatory (in the sense of protective) function. A long-term reduction in food intake, especially extended parenteral feeding, without taking special account of trace elements, leads for all of them, but especially for zinc, to a depletion of body reserves. The latter results in immunological changes that are at first sub-clinical and scarcely recognizable, but which, over the course of time, can lead to life-threatening infections. Cell-mediated immunity, antibody reactions and antibody affinity, complement system and phagocyte activity are perceptibly diminished. The zinc concentration in the blood has an essential influence on the extent and consequences of immunological deficiency.