The author at the entrance of the Liverpool School of Tropical Medicine, 2001  Annals of Tropical Paediatrics (2001) 21,191-194 Taking the HIV Factory Tour HOWARD STEEL ARMISTEAD, BA, MA, Member of the International AIDS Society The immune system is the most complex sys­tem in the human body. It consists of a num­ber of organs and many types of cell. Immune cells travel throughout the body, protecting us from microbes and viruses in the environment and fighting infection. If the immune system is seriously damaged, as in HIV disease, the body loses its ability to stop opportunistic infectious agents from destroying it.   Cells are the basic building blocks of all living things. The human body is a community of billions of cells that have evolved together and work together for common benefit. Immune cells serve as the army that protects this body of cells from harmful invaders. As in any fighting force, communication within the immune system army is crucial. Cells use chemical m...