SummaryDietary sources of nucleic acids and/or their components, have not been considered essential for normal growth and development. However, growing evidence shows that the compounds regulate various steps of the immune system and demonstrate the neccessity of the com pounds in the response to immunological challenge. The significance of exogenously administered purine or pyrimidine bases, nucleotides, and nucleosides in the immune response is reviewed. Key Words nucleoside, nucleotide, infection, immunity, parenteral for mula, enteral formula Nucleic acids and/or their components in the diet have not been considered essential for normal growth and development because it was generally assumed that living organisms, including humans, could synthesize adequate amounts of the compounds required for normal growth and development and that dietary sources are not utilized. This assumption was based on studies done on normal healthy individuals who were not confronted with an immunological challenge (1, 2). Recent studies have documented that the gut and the immune system depend mainly on salvage of purine and pyrimidine bases and that the de novo synthesis is not adequate to meet the demand of various metabolically active cells or tissues. This report reviews current knowledge on the role of nucleic acids and/or their components.Nucleotides are low molecular weight biological compounds that are involved in almost all biochemical processess. They consist of either purines or pyrimidines (nitrogen containing bases). The major purine bases are adenine and guanine, and the major pyrimidines are thymine, cytosine, and uracil. Nucleotides are the building blocks of DNA and RNA and play essential roles in structural, metabolic, energetic, and regulatory functions particularly as components of adenosine tri phosphate and other nucleotide triphosphates and in many co-enzymes. The daily