White blood cells of the type called lymphocytes are able to recognize almost any kind of foreign material that enters the body—including bacteria, viruses (such as HIV), and man-made chemicals that did not exist when the immune system was evolving. Lymphocytes are divided into two principal groups, termed B cells and T cells. Both have the ability to identify a wide array of intruders because each bears on its surface a unique receptor, one created by random combinations of relatively few components. Much as random choices from a restaurant menu can lead to meals with a huge number of variations, random combinations of components can lead, to trillions of different receptors. The human body therefore has at least a trillion ways of recognizing that something foreign has invaded.