Lymphocytes are smaller than phagocytes. They have a large nucleus that fills most of the cell. There are 2 types of lymphocyte, both of which are produced before birth in bone marrow.


B lymphocytes (B Cells) remain in the bone marrow until they are mature and then spread throughout the body concentrating in lymph nodes and the spleen.


T lymphocytes (T cells) leave the bone marrow and collect in the thymus where they mature (Thymus-Gland that lies in the chest just beneath the sternum. It doubles in size between birth and puberty, but after puberty 
it shrinks.



Only mature lymphocytes can carry out immune responses. During the maturation process many different types of B and T lymphocyte develop, perhaps many millions.

Each type is specialised to respond to one Antigen, giving the immune system as a whole the ability to respond to almost any type of pathogen that enters the body.


When mature, all these B and T cells circulate between the blood and the lymph. This ensures that they are distributed throughout the body so that they come into contact with any pathogens and with each other. Immune responses depend on B and T cells interacting with each other to give an effective defence.


Certain T cells co-ordinate the immune response, stimulating B cells to divide and then secrete antibodies into the blood; these antibodies destroy the antigenic pathogens.


Other T cells seek out and kill any of the body’s own cells that are infected with pathogens. To do this they must make direct contact with infected cells.


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