Immunotherapeutics are pharmaceutical products intended to modify immune functions either directly or indirectly by influencing physiological systems that affect immunological functions. They include conventional immunosuppressants, monoclonal antibodies, recombinant cytokines, gene therapy products or therapeutic vaccines. A variety of adverse effects involving the immune system have been described in laboratory animals as well as in the clinic. Some of these adverse effects can be predicted, at least to some extent, from the immunopharmacological profile of these drugs, but a number of unpredicted adverse effects have been described. Immunosuppression, allergy, autoimmunity, immunoactivation are the major adverse consequences to be expected as illustrated by this overview of the leading immunotherapeutics already used in the clinic. These untoward and potentially severe consequences support the need for a careful preclinical and clinical evaluation of the immunotoxicity of these products.