Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
Lymphocytes are the nearly colourless cells found in the blood, lymph, and lymphoid tissues. Approximately a quarter of white blood cells are lymphocytes.
There are two sorts of lymphocyte - "T" and "B".
As T cells differentiate in the thymus, they go through a lot of processing to ensure that they don't develop into cells which will attack "self", i.e. other cells and proteins which are a part of the human body. In fact, T cells of all kinds of specificity develop in the thymus, but they are tested against "self" protein. Any T cells which bind strongly to self protein are destroyed. (In fact, T cells which don't bind *at all* to self protein are also destroyed - because this means they must be defective! A "normal", well-developed T cell will bind to self proteins and any other possible antigen, but only weakly. About 95% of developing T cells are destroyed in the thymus because of these checks.)
The name "B" comes from the fact that scientists (who were studying the immune system of birds) found that these cells were found to mature in the Bursa of Fabricius (a thymus-like organ). The name was then used for this type of cell in all animals. The job of B cells is to be ready to produce antibodies to defend against infection.