Several tobacco antigens have been isolated inducing both precipitating and reaginic antibodies in humans and experimental animals. Chronic smoking leads to histological changes in the respiratory tract. --Alterations of humoral immunity have been demonstrated in the respiratory tract of smokers. Smoking may impair the systemic humoral immunity both in vitro and in vivo. Cell-mediated immunity are altered locally and systematically in smokers. --The clinical incidence of tobacco allergy is not sure, but there are evidences that allergic individuals are more sensitive to the nonspecific noxious effects of active or passive smoking than healthy individuals.