26.11–26.13: Malfunction of the immune system causes disease.

HIV viruses being released from lymph tissue.

Figure 26.28: Type 1 diabetes is an all-too-common autoimmune disorder.

What happens when the immune system malfunctions? Genetic defects, environmental influences, and even viruses can trigger immune dysfunction. And in many cases, such as autoimmune diseases, a combination of causes is suspected, but not completely understood. Autoimmunity occurs when an individual’s immune system responds inappropriately to the individual’s own cells and tissues as if they were pathogens, mistaking “self” for “non-self.” If immunity can be compared to warfare, then autoimmunity is equivalent to an army turning its weapons upon its own citizens. Lymphocyte receptors wrongly recognize an individual’s own molecules or cellular structures as antigens, and the humoral and/or cell-mediated immune responses can be initiated by these antigens. Most autoimmune diseases result in significant tissue and organ damage, and some can even result in death.

Type 1 diabetes (sometimes called juvenile diabetes) is an example of an autoimmune disorder. It is diagnosed in more than 13,000 young people in the United States each year. Symptoms include high blood glucose levels, weight loss, unquenchable thirst, frequent urination, fatigue, and weakness. In this disease, cytotoxic T cells, complement proteins, and macrophages destroy the person’s own pancreatic cells. Without a normal pancreas, a person cannot make insulin (FIGURE 26-28). Without insulin, cells in need of glucose are not able to take it up from the blood, even though glucose is plentiful. In turn, the body breaks down muscle and fat tissues to provide cells with needed energy, and blood glucose levels remain high. The exact cause of this autoimmune reaction is not understood. Treatment for individuals with type 1 diabetes is insulin injections or implantation of an insulin pump, which directly monitors blood glucose levels and releases insulin as needed.

Figure 26.29: An autoimmune disorder: multiple sclerosis.

In some disorders, including type 1 diabetes, the immune system destroys a single cell type or a single organ. In other disorders, the damage is spread throughout the body. When the insulation that surrounds nerve fibers of the brain and spinal cord is under immune attack, the resulting disorder is called multiple sclerosis (MS) (see Section 23-5). Individuals with MS usually begin to exhibit symptoms between 20 and 40 years of age, and these symptoms include blurred vision, weakness in the arms and legs, trouble with balance, and tingling sensations (FIGURE 26-29). This disorder tends to affect women more than men, and it affects more than 300,000 people in the United States. There is no cure for MS, but anti-inflammatory drugs can reduce the severity of some symptoms. Many new drugs are being investigated, including some that block the inflammatory response.

Rheumatoid arthritis is another autoimmune disease that affects women more frequently than men, and it usually strikes between ages 40 and 60. (This form of arthritis is a far less common cause of joint inflammation than osteoarthritis, which is not an autoimmune disease.) The linings of joints are the targets of this immune attack—leading to pain and severe joint swelling that can result in deformities. Various pharmaceuticals are available to help reduce symptoms, and joint replacement surgeries also provide relief for individuals suffering from rheumatoid arthritis.