The sarcoid granulomas and the preceding mononuclear infiltration are composed of immunologically activated cells. Hyperactivity of circulating B cells and the frequent impairment of cell-mediated hypersensitivity could be due to an imbalance between T cell sub-populations. Kveim-Siltzbach test positivity and demonstration of circulating antibodies specifically directed against a sarcoid spleen extract suggest that this extract could have antigenic properties. Cell populations yielded by broncho-alveolar lavage in patients with hypersensitivity pneumonitis or with respiratory sarcoidosis are closely related. It is conceivable that sarcoidosis could result from some inhaled agent(s) which induce a self-perpetuating immunologic response of indeterminate duration. The relevance of immunological studies on sarcoidosis for medical practice is summarized.