HIV destroys a particular variety of white blood cells that are essential for destroying disease-causing germs. There are several varieties of white blood cells in the body. Of these, lymphocytes form about twenty-five per cent of the total white blood cell count. They normally increase in number in response to any infection. There are two types of lymphocytes: (а) В cells and (b) T cells. When the В cells come in contact with a disease-causing agent such as bacteria or virus, they secrete large volumes of antibodies — chemical substances that can destroy the disease-causing germs. The main functions of В cells are to search, identify and then bind with the disease-causing germs.
The T cells are lymphocytes that have travelled through a small gland called the thymus gland, which is situated in the middle and upper part of the bony cage of the chest. When a disease-causing germ enters the body, the T cells produce several new copies of itself. Each T cell contains chemical substances that can destroy the specific disease- causing germs. T cells are also called "killer cells" because of their two main actions, which are (a) they secrete chemical substances necessary for destroying the disease-causing germs and (b) they help the В cells in destroying the agents.