Participants will have a blood sample drawn and will undergo lymphapheresis - a procedure for collecting large numbers of white blood cells called lymphocytes. The blood sample is tested for blood counts and HLA type, a genetic marker of the immune system. HLA molecules help determine the way the body's immune cells respond to virus. HLA typing is similar to blood typing. Usually done to match stem cell or organ transplants, HLA testing may also be used to try to identify factors associated with an increased risk of certain diseases or conditions. HLA type is strongly associated with ethnic background.