When your body encounters a germ, its first line of defense is your immune system. Specialized cells and organs help your body recognize and respond to foreign invaders. Your immune system even has its own circulatory system, called the lymphatic system. If your immune system cannot do its job, the results can be serious. Disorders of the immune system include Allergy and asthma - inappropriate immune responses to substances that are usually harmless Graft-vs.-host disease - a life-threatening reaction in people receiving organ transplants Immune deficiency diseases - disorders in which your resistance to disease becomes dangerously low Autoimmune diseases - diseases causing your immune system to attack your own body's cells and tissues by mistake