Stress can cause illness by impairing the workings of the immune system. The immune system guards the body against attackers, both from within and outside. The white blood cells (leucocytes) within the immune system identify and destroy foreign bodies (antigens) such as viruses. It also leads to the production of antibodies. There are several kinds of white blood cells or leucocytes within the immune system, including T cells, B cells and natural killer cells. T cells destroy invaders, and T-helper cells increase immunological activity. It is these T-helper cells that are attacked by the Human Immuno Deficiency Virus (HIV), the virus causing Acquired Immuno Deficiency Syndrome (AIDS). B cells produce antibodies. Natural killer cells are involved in the fight against both viruses and tumours.