Rhinitis is a disease of the nasal passages that is char­acterized by attacks of sneezing, increased nasal secretion, and stuffy nose (caused by swelling of the nasal mucosa). The symptoms may result from an allergic reaction, or they may be due to a nonspecific hyperactivity of the nasal mucosa (vasomotor rhinitis) in response to environmental irritants. See also Allergic Rhinitis; Hay Fever.

Hypersensitivity pneumonitis is an allergic disease of the lungs caused by inhaling various organic dusts. A person suffering from the disease shows symptoms similar to those of flu: fever, aching, and difficulty breathing. Hypersensitivity pneu­monitis occurs four to six hours after inhalation of the dusts. Pro­longed low-grade exposure to such dusts may result in more insidious illness characterized by gradual onset of shortness of breath at the time of exercise. The exact immune mechanism responsible for the disease has not been established. Antigen-antibody reactions, as well as cell-mediated immune responses, are believed to be implicated as causes of the lung damage that results. Hypersensitivity pneumonitis has numerous causes (see Table 2.2). Although most of the recognized causes are found in the workplace, the disease may be related to a hobby or it may result from exposure to a variety of microorganisms that con­taminate humidifiers and air-conditioner systems. General treatment consists of avoiding further exposure, and severe conditions are treated with corticosteroids (cortisone). Hypoallergenic Agents A hypoallergenic substance is one that is less likely to cause an allergenic reaction; according to the Food and Drug Administration, however, it has not been proved that any prod- uct is, indeed, hypoallergenic, despite claims by manufacturers to the contrary.

Allergic people react to an allergic food when they have reached their allergy threshold. Thus, those with a very low allergy threshold may react when they merely smell the food allergen, whereas those with a very high threshhold may be able to eat substantial amounts of the offending food without experiencing any reaction at all. Factors that may operate, separately or together, to produce allergic symptoms or to bring a person to his or her allergy threshold include : Simultaneous eating of two or more allergenic foods Fatigue Infection Simultaneous contact with two or more allergens (for example, eating an allergenic substance while near an allergenic animal) Seasonal airborne allergies Sensitivity to certain weather conditions Excessive eating at one sitting Excessive consumption of alcohol Over- or undercooking of food (for example, boiled milk may be tolerated, but not whole milk from the container).

Lymphocytes are derived from precursor cells called stem cells that originate in the yolk sac, liver, and bone marrow of the fetus. They circulate in the blood and lymph fluid, finally lodging in the main lymphatic organs. Bone marrow and the thymus are the primary lymphoid organs; the spleen, lymph nodes, and lymphoid masses asso­ciated with the gastrointestinal and respiratory membranes constitute the secondary, or peripheral, lymphoid organs. Secondary lymphoid organs contain an intricate network of col­lagen fibers, to which phagocytic cells are attached and in which lym­phocytes mix thoroughly as they move along. This process permits the lymphoid organs to act as efficient traps for foreign material that has gained access to the body. These organs are also well adapted for interaction between lymphocytes and the foreign matter. The stem cells that remain in the bone marrow after birth are pluripotent; they continue to give rise not only to more stem cells but to all types of red and white blood cells, including lymphocytes. Moreover, they retain this ability as a person matures and becomes an adult. Those stem cells destined to become lymphocytes mature in one of two ways: they become either "T" cells or "B" cells. During the pre- and postnatal parts of their lives, the precursors of T cells migrate from the bone marrow to the thymus (a small organ at the base of the neck), where they establish themselves and become mature T cells. For reasons that are not yet clear, as many as 90 percent of the thymic lymphocytes die within the thymus. It has been hypothesized that these are cells destined to react with "self-antigens," which, by this means, were eliminated in an attempt to avoid an autoimmune reaction. Whatever the reason, some mature T cells eventually leave the thymus and enter the body’s circulatory system. In the blood, they constitute about 75 percent of the circu­lating lymphocytes, or about one-third of all the white blood cells in circulation. The cells can be recognized and counted by virtue of spe­cial "markers" on their surface. For example, human T cells can bind the red blood cells of a sheep to their surface. When human white blood cells are mixed with a sheep’s red blood cells, the sheep’s cells form small clusters around the T cells, which allows ready identifi­cation of the T cells. In a process called cell-mediated immunity, the T cells play a major role in the body’s resistance to viruses, fungi, and intracellular bacteria (for example, those that cause tuberculosis), to protozoan parasites such as those that cause African sleeping sickness, in the rejection of foreign tissue or organ transplants, and in immunity to tumors. When mature T cells are stimulated by an appropriate stimu­lus as an antigen, they proliferate and secrete chemical substances called lymphokines. The most important lymphokines are the macro­phage migration-inhibition factor (MIF), the macrophage-activating factor (MAF), interferon, the T cell-replacing factor, and several fac­tors that attract white blood cells. A macrophage is a scavenger (phagocytic) cell derived from a monocyte, a particular kind of white blood cell. Macrophages are found throughout the body, but they are more likely to be found where there is chronic inflammation. They vigorously ingest and de­stroy numerous foreign substances and microorganisms. In contrast to T cells, which appear to respond to a limited number of antigens, macrophages ingest antigens more or less indiscriminately. In other words, it is the T cells, and not macrophages, that recognize a sub­stance or microorganism as foreign. When a foreign substance invades the body, a group (or subset) of T cells identifies the invader and produces lymphokines, which then bring the macrophages to the site of the invasion. There the macrophages attempt to halt and destroy the foreign substance. The body’s response is the same whether the substance is harmful (in the case of tuberculosis bacteria), helpful (a kidney transplant), or indifferent (poison ivy). There are several different types of T cells in addition to those involved in cell-mediated immunity. One type, known as a cytotoxic T cell, attacks and destroys almost all foreign cells, including tumor cells, which, though not exactly foreign, differ sufficiently from nor­mal cells for the immune system to recognize them as foreign. Two other types of T cell, "helper" T cells and "suppressor" T cells, help regulate the production of antibodies. Lymphocytes, particularly T lymphocytes, can circulate throughout the blood and lymph and peripheral tissues, and then recirculate. They also have a lengthy life span, living as long as twenty years. Because of their circulation ability and longevity, they are superbly equipped to play a major part in the recognition and response functions of adaptive immunity. В cells begin to mature in the bone marrow. At some point, they leave the marrow and migrate to the peripheral lymphatic tissues and organs (Figure 5.1), where they are in a good position to interact with antigens, T cells, and macrophages.

The goosefoot and careless weed (Chemopodiaceae and Amaranthacae) families produce abundant pollen that is highly allergenic. These weeds are particularly im- portant in the Great Plains and the Southwest. The most important member of the group is Russian thistle.

Climate is a combination of many meteorolog­ical forces, including temperature, wind velocity, barometric pressure, and humidity. It strongly affects allergens in the environment, such as pollen, mold, and air pollutants—particularly their concentration and dispersal. For this reason, climate may have a marked effect on peo­ple with respiratory allergies. Pollen-producing plants that grow in a given region do so because of the region’s climate. Ragweed, for example, is the most prominent hay-fever-producing plant in many parts of the United States, but it is not found in the Pacific west or in southern Florida. Warm, humid climates favor the growth of molds. It has long been established that climate affects the concentration of particles in the air, including allergens. For instance, during periods of thermal inversion, there is a marked increase in the amount of pol-









lutants in the air. These pollutants are associated with deleterious health effects, especially among people with respiratory problems.

Following are some suggestions for the relief of less serious allergic reactions: 1. Oral nasal decongestant drugs such as pseudoephedrine may be taken up to four times daily in doses of 30 to 60 milligrams each. 2. Nasal passages may be irrigated with a mild salt solution made with four ounces of warm tap water and a quarter teaspoon of table salt, using a nasal dropper. 3. Nose drops and sprays—for example, Neosynephrine, phen­ylephrine, Afrin, or other long-acting medication—should not be used for more than four consecutive days. Chronic use leads to renewed swelling and nasal obstruction, caused by the drops. Nasal solutions containing Mentholatum and other oils are best avoided. 4. For temporary relief, take hot showers and apply heating pads and hot cloths to the nasal area. Sneezing, Itching, and Watery Discharge of Hay Fever Antihistamines are more effective in relieving the itch and watery discharge phases of hay fever and allergic rhinitis. If a person is taking blood-pressure medications such as reserpine or propanalol, a physician should be consulted as to whether the medications are contributing to the stuffiness. The following suggestions should help relieve symptoms: 1. An antihistamine is the best choice of drug. Chlorphenera-mine, in doses of 2 and 4 milligrams, are obtainable without a prescription. Such side effects as drowsiness, dryness, and gastric disturbance can be combated by varying either the dose or the preparation. Many preparations are combinations of antihistamines and adrenergic decongestants. For rapid, reliable action, a short-acting drug is preferable. Among many people, "sustained-effect" drugs are absorbed less evenly; they may be considered for night­time use after other symptoms have been controlled. 2. Avoidance measures should be considered part of the treat­ment. If the culprit is pollen, exposure should be minimized. Avoid gardening, lawn mowing, and hiking in areas of tall vegetation. Stay indoors during high winds and keep bedroom windows closed when sleeping. If conditions at work produce symptoms, consider such measures as improving the ventilation, installing suction air hoods, and using a respiratory mask.