Hypersensitivity pneumonitis is an immunologically induced lung disease. Although both immune complex-mediated immune response and T cell–mediated immune response are involved in the pathogenesis of the disease, recent studies show the latter mechanism to be more important. As for T cell–mediated immune response, Th1/Th2 and Tc1/Tc2 cytokines produced by CD4+ and CD8+ T cells play important roles in the development of granulomatous inflammation in the lung, a pathologically characteristic feature of the disease. The critical distinction between CD4+ and CD8+ T cells pertains to recognition of antigens presented by different major histocompatibility complex molecules. Serum levels of KL-6 and soluble intercellular adhesion molecule-1 in patients with HP are useful markers of the disease activity. The chronic form of HP can be difficult to diagnose, and provocation testing is helpful. Erythromycin might be useful for anti-inflammatory therapy.