Occupational immunologic lung disease is characterized by an immunologic response in the lung to an airborne agent inhaled in the work environment and can be subdivided into immunologically mediated occupational asthma (OA) and hypersensitivity pneumonitis (HP). Irritant-induced OA, a separate nonimmunologic entity, can be due to chronic exposure to inhaled irritants or reactive airways dysfunction syndrome (RADS). RADS is defined as an asthma-like syndrome that persists for >3 months and occurs within minutes to hours after a single exposure to a high concentration of a respiratory irritant. Workers in high-risk fields for OA include farmers, printers, wood workers, painters, plastics workers, cleaners, spray painters, electrical workers, and health-care workers. OA can be triggered by high-molecular-weight (HMW) proteins that act as complete allergens or by low-molecular-weight (LMW) sensitizers that act as haptens. HMW proteins (>10 kDa) are generally derived from microorganisms (such as molds and bacteria, including thermophilic actinomycetes), plants (such as latex antigens and flour proteins), or animals (such as animal dander, avian proteins, and insect scales) and are not specifically regulated by the Occupational Safety and Health Administration. LMW haptens that bind to proteins in the respiratory mucosa include some Occupational Safety and Health Administration regulated substances, such as isocyanates, anhydrides, and platinum. HP can present in an acute, chronic, or subacute form. The acute, subacute, and early chronic form is characterized by a CD4+ T-helper type 1 and CD8+ lymphocyte alveolitis. Classically, the bronchoalveolar lavage in HP will show a CD4/CD8 ratio of <1.