T and B lymphocytes are the cells of the adaptive immune system. The human body has about 2 trillion lymphocytes, which are 20–40% of white blood cells; their total mass is about the same as the brain or liver. The peripheral bloodstream contains only 2% of all circulating lymphocytes; the other 98% move within tissues and the lymphatic system, which includes the lymph nodes and spleen. In humans, approximately 1–2% of the lymphocyte pool recirculates each hour to increase the opportunity for the cells to encounter the specific pathogen and antigen that they react to.