Lymphocytes, unlike phagocytes, act against specific pathogens. Each lymphocyte contains a set of genes that codes for the production of a particular type of receptor. We have many million different types, each producing just one type of receptor.
Both B-lymphocytes and T-lymphocytes are made in bone marrow. B-lymphocytes then spread through the body and settle in lymph nodes, although some continue to circulate in the blood. T-lymphocytes collect in the thymus gland, where they mature before spreading into the same areas as B-lymphocytes. The thymus gland disappears at around the time of puberty.
Both types of lymphocyte have a large, rounded nucleus that takes up most of the cell. They can only be told apart by their different actions.
During the maturation process, any lymphocytes that produce receptors that would bind with those on the body's own cells are destroyed. This means that the remaining lymphocytes will only act against non-self molecules that are not normally found in the body. Non-self molecules, such as those on the surfaces of invading bacteria, are called antigens.