In this monograph, the function and histophysiology of the immune system are reviewed, and the information necessary to understand and interpret the pathological changes caused by immunotoxic insults is provided. Emphasis is laid on the immune systems of humans and rodent species, but reference is made to other species, including fish, that have been the object of immunotoxicological studies. The patho physiology of the immune system, including the variable susceptibility of its components, alterations to the lymphoid organs, and the reversibility of changes are important for understanding the impact of immunotoxicity. This monograph is concerned mainly with one aspect of immunotoxicology: the direct or indirect effect of xenobiotic compounds (or their biotransformation products) on the immune system. This effect is usually immunosuppression, or the induction of a state of deficiency or unresponsiveness. Allergy and autoimmunity will be dealt with in a future Environmental Health Criteria monograph.