THE LIVING WORLD
Unit Six. Animal Life
27. How the Animal Body Defends Itself
Although the immune system is one of the most sophisticated systems of the vertebrate body, it is still not perfect. Many of the major diseases we face, and some minor irritations as well, reflect an overactive immune system.
The ability of T cells and B cells to distinguish cells of your own body—“self” cells—from nonself cells is the key ability of the immune system that makes your body’s third line of defense so effective. In certain diseases, this ability breaks down, and the body attacks its own tissues. Such diseases are called autoimmune diseases.
Multiple sclerosis is an autoimmune disease that usually strikes people between the ages of 20 and 40. In multiple sclerosis, the immune system attacks and destroys a sheath of fatty material, called myelin (see chapter 28), that insulates motor nerves (like the rubber covering electrical wires). Recall from section 22.6 that nerve impulses travel along the length of the nerve cell, and so degeneration of the myelin sheath interferes with transmission of nerve impulses, until eventually they cannot travel at all. Voluntary functions, such as movement of limbs, and involuntary functions, such as bladder control, are lost, leading finally to paralysis and death. Scientists do not know what stimulates the immune system to attack myelin.
Another autoimmune disease is type I diabetes in which cells are unable to take in glucose because the pancreas fails to produce insulin (recall from chapter 26 that insulin plays a key role in the liver’s regulation of levels of glucose in the blood). Type I diabetes is thought to result from an immune attack on the insulin-manufacturing cells of the pancreas. No one knows why the attack occurs. Other autoimmune diseases are rheumatoid arthritis (an immune system attack on the tissues of the joints), lupus (in which the connective tissue and kidneys are attacked), and Graves’ disease (in which the thyroid is attacked).
Although your immune system provides very effective protection against fungi, parasites, bacteria, and viruses, sometimes it does its job too well, mounting an immune response that is greater than necessary to eliminate an antigen. The antigen in this case is called an allergen, and such an immune response is called an allergy. Hay fever, sensitivity to even tiny amounts of plant pollen, is a familiar example of an allergy. Many people are also allergic to nuts, eggs, milk, penicillin, and even proteins released from the feces of the house dust mite (figure 27.18). Many people sensitive to feather pillows are in reality allergic to the mites that are residents of the feathers.
Figure 27.18. The house dust mite.
This tiny animal, Dermatophagoides, causes an allergic reaction in many people.
What makes an allergic reaction uncomfortable, and sometimes dangerous, is the involvement of antibodies attached to a kind of white blood cell called a mast cell. It is the job of the mast cells in an immune response to initiate an inflammatory response. Figure 27.19 shows what happens when a mast cell encounters something that matches its antibody. Mast cells release histamines and other chemicals that cause capillaries to swell. Histamines also increase mucus production by cells of the mucous membranes, resulting in runny noses and nasal congestion (all the symptoms of hay fever). Most allergy medicines relieve these symptoms with antihistamines, chemicals that block the action of histamines.
Figure 27.19. An allergic reaction.
In an allergic response, B cells secrete IgE antibodies (see page 572) that attach to the plasma membranes of mast cells, which secrete histamine in response to antigen-antibody binding.
Asthma is a form of allergic response in which histamines cause the narrowing of air passages in the lungs. People who have asthma have trouble breathing when exposed to substances to which they are allergic.
Key Learning Outcome 27.10. Autoimmune diseases are inappropriate responses to "self" cells, whereas allergies are inappropriate immune responses to harmless antigens.