The immunology of the lung in both health and disease has been a major focus of research in recent years. As a result, many pulmonary syndromes previously defined by clinical and pathological descriptions can now be categorized by immunologic processes that appear to contribute to the abnormal state. In this review I examine recent studies of hypersensitivity lung disease, concentrating on observations involving both human disease and animal models of disease. I emphasize the evolution of our understand ing of basic immunologic reactions within the lung. Rather than list all pulmonary disorders where immunologic mechan isms may contribute to the process, I stress general concepts and discuss the more common lung diseases. It is apparent that the disease processes are complex, that our knowledge in many areas is incomplete, and that no simple method of categorizing lung diseases with an immunologic basis is possible.

The lack of a satisfactory system of classification of hypersensitivity reactions deserves early comment. The classification of allergic reactions introduced by Coombs & Gell is now over 20 years old (1). While the actual reactions are more complex than the four types initially described, and although the categories tend to oversimplify pathologic states when applied to human disease, the Coombs & Gell scheme does offer a basic framework for discussion and is therefore employed in this review. In addition, examination of how our thinking about these reactions has changed over two decades helps us appreciate the evolution of knowledge in the fields of immunology and lung disease.