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In some individuals, the immune system may not work adequately or may work against its own body tissue. Such conditions are called either immune-mediated (the immune system allows damage to take place without intervening) or autoimmune (the immune system mistakenly attacks its own tissues).

Some immune-mediated conditions include:

• Allergic reaction. Caused when the immune system reacts to a non-threatening substance (an allergen) as if it were dangerous. The body attempts to remove the threat, causing symptoms such as sneezing, a runny nose, coughing, itching, nausea, diarrhea or shortness of breath.
  
  
• Allergic asthma. Condition where allergens (e.g., pollen, dander, mold) trigger an asthma attack. When inhaled, the allergens cause inflammation of the airway passages in the lungs, resulting in coughing, wheezing and shortness of breath.
  
  
• AIDS (acquired immune deficiency syndrome). Condition caused by HIV (human immunodeficiency virus) where the immune system becomes suppressed and is unable to fight infections. Bacteria and viruses that are normally repelled by a healthy immune system cause infections that may be life-threatening.

Some of the major autoimmune conditions include:

• Type 1 diabetes. Occurs when insulin-producing cells in the pancreas are mistaken as antigens by the immune system, making it difficult for the body to convert food into energy.
  
  
• Rheumatoid arthritis. Condition in which the immune system causes an inflammation of the membrane lining the joints. It often results in pain and stiffness in the joints.
  
  
• Multiple sclerosis. Disease that results when the immune system damages the nerve tissues in the central nervous system.
  
  
• Inflammatory bowel diseases. Multiple conditions may result from the immune system working against the intestines. These include Crohn disease and ulcerative colitis.
  
  
• Lupus erythematosus. Occurs when the immune system attacks and damages the kidney, brain or lungs.
  
  
• Psoriasis. Condition in which the immune system sends signals that cause skin growth to speed up, resulting in the itchy, flaky buildup of skin.
  
  
• Scleroderma. This disease results from the immune system causing the skin and blood vessels to thicken, resulting in a loss of movement and shortness of breath.
  
  
• Autoimmune thyroid diseases. Conditions in which the immune system damages or stimulates the tissues of the thyroid. Conditions of this type include Hashimoto’s thyroiditis and Grave’s disease.
  
  
• Sjögren syndrome. Occurs when the immune system reacts against some of the body’s own tissues, damaging the glands that produce tears and saliva and causing dry mouth and dry eyes.

Immune system activity plays a major role in organ and tissue transplantation. The immune system functions as it should in such cases because it reacts to the new tissue as a foreign invader. However, for transplantation, such a response must be circumvented so the body will accept the new organ or tissue.  For this reason, transplant patients are often required to take medications (immunosuppressants) to reduce the ability of the immune system to work.