Peripheral blood mononuclear cell samples are collected periodically for research studies. Samples are analyzed to assess the frequency of leukocyte subsets (including B cells, T cells, NK cells, and monocytes) via flow cytometry; frequency of T-regs (T cells that express CD4, CD25, and FoxP3); and responses to keyhole limpet hemocyanin and tetanus toxoid via ELISA assay. Other immunological studies are also performed.