In order to carry out in vitro research procedures on the plasma or leukocyte components of blood, it is often necessary to obtain larger quantities of plasma, leukocytes, or platelets than can be safely obtained by simple phlebotomy. These components can be easily and safely obtained using apheresis procedures in the Clinical Center Apheresis Unit. This protocol is specifically designed to conform to the requirements of the Apheresis Unit for donors to have apheresis procedures (greater than 7 years of age and greater than or equal to 25 kg) but the protocol in itself is not a research protocol. Donors must first be admitted to another approved clinical research protocol of the NIAID before they may have the apheresis procedures described in this protocol.