Immunoglobulin G--which can be subdivided into four classes, each with different functional characteristics--is an important component of the host defense system of the respiratory tract. An excessive amount can be produced or can accumulate after airway irritation (exposure to cigarette smoke) or from immunologic stimulus of B-lymphocyte-plasma cells in types of hypersensitivity and interstitial lung diseases. Specific antibody activity can be identified in organic dust-induced hypersensitivity pneumonitis and asthma that contributes to disease pathogenesis. The availability of opsonic antimicrobial antibodies is essential for optimal function of phagocytes in uptake and containment of bacteria. With an absolute or functional deficiency of IgG, recurrent and chronic types of sinopulmonary infections occur. These extremes of IgG availability, either high levels (presumably excessive) or deficient, are discussed in this review.