Genetic Basis of Immunodeficiency

This study will examine the role of hereditary factors in different forms of severe combined immunodeficiency (SCID). Patients with immunodeficiencies may be eligible for this study. Candidates include: * Patients with diminished numbers of T cells or NK cells or both, or * Patients with normal T cell and NK cell numbers but diminished T cell, B cell, or NK cell function. Relatives of patients will also be studied. Participants will have blood samples collected for genetic analysis in studies related to SCID at the National Institutes of Health and other institutions.