 Outcome Measures: We will measure TRECs in dried blood spots, and correlate TREC number with diagnosis and clinical and laboratory data. We hope to determine the range of immunodeficiency diseases that can be demonstrated to have significantly reduced TRECs as compared to healthy subjects using our assay. Because TREC testing may be insufficiently sensitive or specific as a stand-alone test, further tests may be developed and performed on the samples in the future, including DNA re-sequencing, detection of mRNA for T cell specific genes andor proteins expressed only in T cells. 