Lymphocytes

Lymphocytes are smaller than monocytes and have large nuclei. They circulate in the blood and are present in great numbers in lymphatic tissue such as lymph nodes and the spleen. Lymphocytes develop from pluripotent stem cells in red bone marrow and from precursors in lymphoid tissue, then travel in the blood to lymphoid tissue elsewhere in the body where they are activated, i.e. they become immunocompetent which means they are able to respond to antigens (foreign material). Examples of antigens include:

• cells regarded by lymphocytes as abnormal, e.g. cells that have been invaded by viruses, cancer cells, tissue transplant cells

• pollen from flowers and plants

• fungi

• bacteria

• some large molecule drugs, e.g. penicillin, aspirin.

Although all lymphocytes originate from one type of stem cell, when they are activated in lymphatic tissue, two distinct types of lymphocyte are produced -

T-lymphocytes and B-lymphocytes. The specific functions of these two cell types are discussed in Chapter 15.

Platelets (thrombocytes)

These are very small non-nucleated discs, 2 to 4 μm in diameter, derived from the cytoplasm of megakaryocytes in red bone marrow (Fig. 4.2). They contain a variety of substances that promote blood clotting, which causes haemostasis (cessation of bleeding).

The normal blood platelet count is between 200 × 10⁹/1 and 350 × 10⁹/1 (200 000 to 350 000/mm³). The control of platelet production is not yet entirely clear but one stimulus is a fall in platelet count. The kidneys release a substance called thrombopoietin, which stimulates platelet synthesis; other cytokines may also be involved.

The lifespan of platelets is between 8 and 11 days and those not used in haemostasis are destroyed by macrophages, mainly in the spleen. About a third of platelets are stored within the spleen rather than in the circulation; this is an emergency store that can be released as required to control excessive bleeding.