Until recently, primary immunodeficiency disorders in man have been classified as to whether they appear to involve cellular immunity, humoral immunity, or both together. However, many disorders do not fit easily into such classification. The discovery of specific inherited molecular defects which give rise to immunodeficiency syndromes has provided an alternative approach. Further elucidation of specific molecular defects, as well as of the genetic heterogeneity within each of the known defects, would provide for a clearer understanding of abnormal immune function. Elucidation of the pathogenetic mechanisms involved in the development of immunodeficiency may provide tools for the metabolic manipulation of the normal and abnormal immune system. Three specific (adenosine deaminase, purine nucleoside phosphorylase and transcobalamin II deficiency) and possibly a fourth (5'-nucleoside deficiency) have been described. Three of these defects involve the purine metabolic pathway (crucial for synthesis of the substrates for DNA synthesis) and result in accumulation of potentially toxic metabolites such as cAMP, dATP and dGTP. We will therefore determine the extent of genetic heterogeneity in ADA deficiency. We will develop in vitro and in vivo models of immune function to define the consequences of deficiency of ADA and PNP and test the effectiveness of enzyme replacement versus other metabolic manipulations in reversing these consequences. We will characterize and determine activity of several enzymes of the purine pathway in patients with immunodeficiency of unknown etiology and also determine the presence of abnormalities of nucleoside and nucleotide concentrations in cells and body fluids of such patients.