Most people take their immune system for granted, little realizing that every day there are white blood cells and antibodies busily scrutinizing every corner of the body for foreign material. Immunology is the study of this dynamic and complex system. Immunologists perform laboratory analyses which demonstrate antibodies to both past and present disease and which can determine if a vaccination has been effective. They can also test, classify and count immune system cells to determine the ability of a person to mount an effective immune response, and these types of assays are important in many diseases such as, for example, AIDS. As the rate and success of organ and tissue transplantation skyrockets, immunologists are also busy determining donor and recipient compatibility and the rejection status of transplants. As knowledge of the immune system continues to grow, the career opportunities for clinical immunologists also expand into ever more diverse areas.