This study will collect blood plasma and white blood cells from individuals using a procedure called apheresis. Apheresis is a method of collecting larger quantities of certain blood components that can safely be collected through a simple blood draw. The blood components will be used in laboratory research studies. Patients 18 years of age and older who are currently enrolled in a NIH clinical research protocol may participate in this study. Relatives of patients and normal healthy volunteers will also be enrolled. Individuals will undergo one of the following two apheresis procedures: * Automated pheresis Blood is drawn through a needle placed in an arm vein and circulated through a cell separator machine. The plasma (liquid part of the blood) and white cells are extracted, and the red cells are re-infused into the donor through the same needle or a needle in the other arm. An anticoagulant (medication to prevent blood from clotting) is usually added to the blood while in the machine to prevent it from clotting during processing. * Manual pheresis One unit (1 pint) of blood is drawn through a needle placed in an arm vein, similar to donating a pint of whole blood. The red blood cells, with or without plasma, are separated from the rest of the blood and returned to the donor through the same needle. Manual pheresis will be done only when a person s estimated total blood volume or red cell count is too low to safely permit removal of blood through a pheresis machine. An adult small in size or markedly anemic, for example, may fall into this category. Some of the blood collected through apheresis may be stored for future studies of HIV disease and immune function and for HLA testing, a genetic test of markers of the immune system. Some of the blood may be used to screen for different types of viral liver infections, such as hepatitis A, B, C, D, E, F, or G.