Antigen-specific T cells (ASTC) are T cells that have been activated by a specific antigen/peptide derived from viruses, bacteria, tumor cells, and in the case of autoimmune disease, peptides derived from normal tissues. Recognition of unique peptide sequences leads to structural rearrangement of the T cell receptor and creates an activated population of T cells specifically directed against the distinctive peptide antigen. Each unique peptide is recognized by a small fraction of circulating T cells, making study of antigen-specific reactions difficult. Therefore, ASTC provide a unique tool for studying the activation and specificity of the immune system.