Abstract: A large number of xenobiotics (i. e. chemicals, drugs and biologicals) may adversely affect the immune system in consequence of different mechanisms: 1) the capacity of a large variety of xenobiotics to cause an allergic sensitization and different types of allergic sequelae (e.g. bronchial asthma, rhinitis, hypersensitivity pneumonitis, contact dermatitis); 2) the adjuvant effect, potentiating the IgE mediated immuno-response against ubiquitous allergens (e.g. pollens); 3) the direct or indirect action on the immune system, at doses that did not cause overt toxicity. This effect is studied by a new discipline, called Immunotoxicology; 4) the indirect action on the target organ, which facilitate the onset of respiratory symptoms in previously asymptomatic subjects. The widespread distributions of xenobiotics in the environment suggests that our current knowledge of their adverse health effects may represent only the tip of the iceberg and that exposure to such immunotoxic agents may play a greater role than heretofore suspected in disease causation. The exposure to immunotoxic agents in the workplace might represent and additional risk to HIV positive individuals.