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White blood cells are responsible for protecting the body from invasion by foreign substances such as bacteria and viruses. The majority of white blood cells are produced in the bone marrow, where they outnumber red blood cells by 2 to 1. However, in the blood stream, there are about 600 red blood cells for every white blood cell. There are several types of white blood cells. Granulocytes and macrophages protect against infection by surrounding and destroying invading bacteria, and lymphocytes aid in the immune defense. Granulocytes are prepared by apheresis or by centrifugation of whole blood. They are transfused within 24 hours after collection and are used for infections that are unresponsive to antibiotic therapy. The effectiveness of white blood cell transfusion is still being investigated.
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