Infectious Disorders

Acquired immunodeficiency syndrome (AIDS) is a viral disease that is approaching epidemic proportions. It is caused by the human immunodeficiency virus (HIV), which attacks and kills helper T cells and invades macrophages, which serve as a reservoir for the virus. In time, the immune defenses of the victim are greatly reduced, and the patient becomes susceptible to opportunistic diseases that ultimately lead to death. These secondary diseases include pneumonia caused by Pneumocystis carinii and a cancer known as Kaposi sarcoma, disorders that are rarely encountered except in AIDS patients. At present, there is no cure for AIDS, although major research efforts have provided drugs to slow its progress.

Although HIV has been found in most body fluids, it appears that sufficient HIV concentrations for transmission to other persons are not present in tears or saliva. Transmission does not occur through routine, nonintimate contact; instead, it primarily occurs through blood exchanges and sexual intercourse. Transmission occurs through exchanges of blood, most commonly by the use of contaminated hypodermic needles and by exposure of open wounds or mucous membranes to infected blood. Vaginal fluids and semen of infected persons are effective transmitting agents in sexual intercourse. Also, infected mothers may transmit HIV to infants during childbirth.

Elephantiasis (el-e-fan-ti ‘-ah-sis) is a chronic condition characterized by greatly swollen (edematous) lower limbs or other body parts, which become “elephantlike” in appearance. This occurs because the lymphatic vessels are blocked by masses of microscopic roundworms, which causes fluid to accumulate excessively in the tissues drained by the plugged lymphatic vessels. The microscopic worms are transmitted by the bites of certain species of mosquitoes found in tropical regions.

Tonsilitis is the inflammation of the tonsils. It usually results from bacterial infections that cause the tonsils to become sore and swollen. If this condition becomes chronic and interferes with breathing or swallowing, or becomes a persistent focal point for spreading infections, the tonsils may be surgically removed. Tonsillectomies are much less common now that effective antibiotics are available and the role of tonsils in immunity is better understood.

Lymphadenitis (lim-fad-en-ahy’-tes) is inflammation of the lymph nodes. It is a common complication of bacterial infections and is often described as “swollen glands.” Doctors often feel the cervical region for swollen lymph nodes to see if they are inflamed due to fighting an infection in the head or neck, which drains into the cervical lymph nodes.

A severely swollen lymph node is called a bubo. Buboes are named after the characteristic swelling of the inguinal and axillary lymph nodes in individuals infected with the bubonic plague, a bacterial infection of the lymphoid system that contributed to the Black Death. The Black Death is the largest known pandemic (large infectious outbreak), which killed about half of the population of Europe (estimated at up to 200 million people worldwide) from 1347 to 1351.

Non infectious Disorders

Allergy, or hypersensitivity, refers to an abnormally intense immune response to an antigen that is harmless to most people. Such antigens are called allergens to distinguish them from antigens associated with disease. Once a person is sensitized to an allergen, an allergic reaction results whenever subsequent exposure to that allergen occurs. Allergic reactions may be either immediate or delayed.

Immediate reactions result when allergens bind with IgE on the surface of mast cells. This interaction causes these cells to secrete substances-such as histamine-that stimulate an inflammatory response. Immediate allergic reactions may be either localized or systemic (whole body). Localized reactions, such as hay fever, hives, allergy- based asthma, and digestive disorders, are unpleasant but rarely life threatening. In contrast, systemic allergic reactions, also known as anaphylaxis, are often life threatening. They quickly impair breathing and may cause circulatory failure due to a sudden drop in blood pressure as blood vessels dilate and fluid moves into the tissues. Allergic reactions to penicillin and bee stings can cause systemic allergic responses.

Delayed allergic reactions appear one to three days after exposure to the antigen. Delayed allergic reactions result from cytokines released by T cells. The dermatitis that occurs following contact with poison ivy and some cosmetic chemicals is a common delayed allergic reaction.

Autoimmune diseases result when T and B cells, for unknown reasons, recognize certain body tissues as foreign antigens and produce an immune response against them. This problem may result because certain body molecules have changed slightly and are no longer recognizable as self. Some of the most common autoimmune diseases are

  • rheumatoid arthritis (RA), which destroys joints;
  • type I diabetes, which destroys beta cells in the pancreas;
  • multiple sclerosis (MS), which destroys the myelin sheath in the CNS;
  • Graves disease, which stimulates the thyroid gland to produce excessive amounts of thyroid hormones;
  • myasthenia gravis, which impairs nerve impulse transmission at neuromuscular junctions; and
  • Hashimoto disease, which destroys the thyroid gland, creating hypothyroidism. It is the most common cause of hypothyroidism in the United States.

    Lymphoma is a general term referring to any tumor of lymphoid tissue. There are several types of lymphomas. One type of malignant lymphoma is Hodgkin lymphoma, which is cancer of lymphoid tissue involving the production of B cells. It is characterized by lymphadenitis, fatigue, and sometimes fever and night sweats. Early treatment with chemotherapy or radiation yields a high cure rate.

    Severe combined immunodeficiency (SCID) is a group of disorders resulting from several different genetic defects. They are characterized by a marked deficit or absence of both T cells and B cells. Thus, the lymphoid system of affected individuals is basically nonfunctional. Infants have little or no protection against pathogens and usually die within a year without treatment. Transplants of normal stem cells from red bone marrow or umbilical cord blood have proved to be successful in some cases, and gene therapy trials look promising for treating this devastating disease.