The frequency and distribution of B and T lymphocyte subsets have been determined in many body tissues and fluids by preparing cell suspensions and tissue sections from lymphoid and nonlymphoid organs. In humans these studies often concentrate on the blood or on one particular cell source for obvious reasons. However, such data can only be interpreted correctly if the whole immune system is taken into consideration [64]. To facilitate this, reports on the frequencies and the absolute numbers of B and T lymphocyte subsets within various human tissues and fluids have been collected from a wide variety of journals and are briefly summarized here. Since the size of lymphoid organs varies with age (e.g. thymus, tonsils), only the data of adult individuals were included, unless otherwise stated. Natural killer (NK) cells are morphologically quite similar to lymphocytes [59], but very different functionally. For example, they are not able to recirculate from the blood via the lymph nodes and the thoracic duct back to the blood as lymphocytes do [19]. Thus, human NK cells have been compared with lymphocytes with respect to number and distribution.