White blood cell

The blood contains erythrocytes (red blood cells), leukocytes (white blood cells), platelets and plasma.

White blood cells help to defend the body against infectious disease and foreign materials as part of the immune system.

White blood cells are also called leukocytes or immune cells. As well as in the blood, white cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues.

A type of cancer in which white blood cells multiply out of control is called leukemia, of which there are several types.

There are many types of white blood cell:

Granulocytes

There are three types of granulocytes: neutrophils, basophils and eosinophils, named according to their staining properties.

• Neutrophils, so-called because they stain neutral, contain multilobed nuclei, normally 2-5 lobes. They are the primary cell of the acute inflammatory response, and act by phagocytosing foreign material and pathogens. Neutrophils are like the 'vacuum cleaners' of the immune system, engulfing and destroying pathogens, or tissue debris after injury.
• Eosinophils, so-called because they stain brick-red with the more acidic stain, known as eosin. These cells have a role in the allergic response, and in defending against parasites such as worms.
• Basophils, so-called because they stain dark-blue with the more basic stain. These cells are the least common circulating white blood cell, and their purpose is not that clear. Though it is thought that these cells become tissue mast cells.

Other White Cells

• Lymphocytes, much more common in the lymphatic system, and including the so-called "killer T-cells" The blood has three types of lymphocytes: B cells, T cells and natural killer cells. B cells make antibodies that bind to pathogens to enable their destruction. CD4+ (helper) T cells co-ordinate the immune response (they are what become defective in an HIV infection). CD8+ (cytotoxic) T cells and natural killer cells are able to kill cells of the body that are infected by a virus.

• Monocytes share the 'vacuum cleaner' function of neutrophils, but are much longer lived as they have an additional role. Monocytes, and their tissue counterpart macrophages, present pieces of pathogens to T cells so that they may be recognised again and killed, or so that an antibody response may be mounted.

Other Tissue Cells

• Histiocytes, found in the lymphatic system and other body tissues, but not normally in blood:
  • Macrophages
  • Dendritic cells
• Mast cells