Lymphocytes are much smaller than monocytes but also have a large single nucleus. Like all blood cells, they circulate in the bloodstream but are found in very large numbers in lymphatic tissue such as the lymph nodes and the spleen.

Like all blood cells, lymphocytes develop from stem cells in the red bone marrow and also precursors in lymphoid tissue. However, they are only activated (ie become immunocompetent which means they are able to respond to antigens and foreign material) when they travel in the blood to lymphoid tissue elsewhere in the body.

Their major roles are strongly related to the basophils and work to identify and respond to allergens. Examples of such allergens include;

• Cells regarded by lymphocytes as abnormal, e.g. cells that have been invaded by viruses, cancer cells, transplanted tissue
• Pollen from flowers and plants
• Fungi
• Bacteria
• Some large molecule drugs, e.g. penicillin, aspirin.

Lymphocytes though all developing from the same type of stem cell develop into two distinct types of lymphocytes in the lymphoid tissue. These are the T- lymphocytes and B- lymphocytes.

Subsequent articles will talk about the types of lymphocytes and their roles in defence and immunity.