After an immune response has run its course, some B-cells remain as memory cells that can respond quickly to future attacks by the same invader. This is why most people catch diseases like chickenpox only once. They are produced in the bone marrow, but they migrate and mature in the thymus , an immune system organ in the chest. When HIV attacks and kills helper T-cells, this coordination is disrupted, and various immune system functions may be disabled. The various components of the immune system communicate with each other using chemical messengers called cytokines .
Macrophages also produce cytokines.
Immune System Problems Because the immune system is so complex, many things can go wrong. There are several hereditary conditions in which a person is born with a defective immune system. In other cases, components of the system may be damaged later in life, leading to acquired immune deficiency. This is what happens in HIV disease. The virus attacks and kills the helper T-cells that coordinate the immune response.