Functions of white blood cells

These depend on the cell type:

* Neutrophils phagocytose bacteria, fungi, protozoa, viruses, foreign cells, tumour cells and toxins. Chemotaxis occurs in response to activated complement proteins, cytokines and microbial products.

* Eosinophils phagocytose antigen-antibody complexes and larval forms of helminthic parasites. They inactivate mediators of anaphylaxis.

* Basophils possess IgE receptors and release mediators from granules in inflammatory and allergic reactions.

* Lymphocytes.

B lymphocytes synthesise antibodies (immunoglobulins), contributing to humoral immunity. In response to a specific antigen they give rise to a monoclonal proliferation of plasma cells, which produce specific antibodies. T lymphocytes, which comprise 65%-80% of circulating lymphocytes, are responsible for cell-mediated immunity. They mediate the cellmediated response to intracellular parasites, viruses, bacteria and fungi; delayed hypersensitivity; graft-versus-host reactions; and organ transplant rejection. They include subpopulations, which can be distinguished by cell surface markers.

Helper (CD4 marker): enhance antibody production by B cells and stimulate the activity of other T cells. Cytotoxic (CD8 marker): kill virus infected and tumour cells, based on previous experience. Suppressor: block helper T cells.

* Monocytes are phagocytic and become macrophages in the tissues. Macrophages occur in the following locations:

Connective tissues: histiocytes

Liver: Kupffer cells

Lungs: alveolar macrophages

Lymph nodes: free and fixed macrophages

Spleen: free and fixed macrophages

Bone marrow: fixed macrophages Skin: Langerhans cells; histiocytes Serous fluids: pleural and peritoneal macrophages

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