The significance of vitamin A and of its metabolites (retinoic acid a.
o.) for the functional capacity of the cells of the immune system (mac
rophages, T- and B-lymphocytes, killer-cells a.o.) is discussed with s
pecial consideration to that of the gastro-intestinal-tract and of the
mammary gland. Recommendations for the application of vitamin A in co
mbination with the vitamins D, E and C are given.