Although large numbers of diseases are primary to the oral cavity, periodontal diseases, by far, comprise those most frequently encountered. Periodontal diseases are infections, but host factors are important determinants of susceptibility and disease progression. Some forms of periodontitis, especially those seen in children and young adults, are related to abnormalities in host defense systems or to systemic diseases. The presence of periodontitis, especially in young individuals, conveys important information about general health. Periodontitis is recognized clinically by inflammation of the gingiva, bleeding, the presence of periodontal pockets, exudation, and tooth mobility. These diseases generally are painless until near their terminal stages. Periodontitis affects approximately 75% of adults under age 65, and almost all older individuals. Approximately 10% of adult Americans have severe periodontitis. Prevalence in children is lower. Several different kinds of periodontitis exist, and these differ in their etiology, natural history, and response to therapy. Prepubertal periodontitis affects the primary teeth of young children. The generalized form, observed in all children with LAD syndrome, rapidly results in loss of the teeth. The localized form, which is far less severe, is indicative of defective PMN chemotaxis. Juvenile periodontitis is observed in teenagers and begins at the circumpubertal period. Affected individuals manifest an abnormality in a surface glycoprotein designated GP110 and an abnormally low number of cell surface receptors for chemoattractants on the phagocytic cells. Abnormal PMN cell and monocyte motility also are observed in patients with rapidly progressive periodontitis. At this time, adult periodontitis, the most frequently observed type, has not been associated with abnormalities in host defense systems. Immunologic studies have been focused on patients with juvenile periodontitis and rapidly progressive periodontitis. Some of these patients mount a humoral immune response, whereas, for reasons that remain unknown, others do not. The IgG antibodies produced during infection are of relatively low avidity and predominated are by IgG2, a subclass considered to be relatively ineffective in opsonizing and enhancing phagocytosis and killing of bacteria by PMN cells. Following periodontal therapy, antibodies that are more highly avid and more effective in enhancing phagocytosis and killing are produced. The innate capacity of an individual to mount a robust humoral immune response may be a major determinant of susceptibility to periodontal diseases.