Antigen

Antigen, any substance that evokes the production of an antibody when introduced into the body of an animal. Antigens can enter the body through the respiratory tract, digestive tract, skin, or blood vessels. The most common antigens are proteins such as those found in bacteria and viruses. See also Allergy.

Anticoagulant

Anticoagulant is any of a group of drugs that suppress, delay, or prevent clots (masses of blood cells) from forming in the bloodstream. Also known as blood thinners, anticoagulants are used to treat heart, lung, or blood vessel disorders. The drug may be administered for a heart attack, pulmonary embolism (blockage of the artery that feeds the lung), or irregular heartbeat, which may cause harmful clots to form. Patients fitted with artificial heart valves may also be treated with anticoagulants following surgery in order to prevent complications associated with blood flow into and away from the heart.

Anticoagulants work by interfering with the action of vitamin K, an essential component in the production of blood clots. Anticoagulants do not dissolve existing clots, which require other forms of treatment.

Anticoagulants are prescription drugs available as tablets taken orally, and in a liquid form that is injected into the skin. An initially high dosage (10 to 250 mg per day, depending on the specific drug) is typically reduced after a few days to a maintenance dosage. The exact dosage is largely dependent on the timing of each patient’s blood-clotting process, as determined by laboratory tests. Effectiveness is usually apparent after three to five days of treatment. One specific type of anticoagulant is warfarin.

Anticoagulants can react adversely with dozens of other drugs including over-the-counter medications such as aspirin and acetaminophen. Anticoagulants also can cause excessive bleeding. Patients taking anticoagulants are often advised to carry an identification card that indicates their treatment with these potent drugs. The use of anticoagulants during pregnancy is not recommended.

Antacids

Antacids, drugs that diminish abnormal acidity in the digestive tract or in the various secretions of other organs. Those that unite directly with free acid in the stomach are known as direct antacids. These include ammonium carbonate, magnesia, potassium bicarbonate, limewater, and chalk. So-called remote antacids, such as the acetates, citrates, and tartrates of the alkalies, act by being changed into carbonates, thus increasing the alkalinity of the blood. Some drugs, such as the carbonates or bicarbonates of sodium, potassium, calcium, magnesium, and lithium, act as both direct and remote antacids.

Amebiasis

Amebiasis, widespread human disease in tropical regions, resulting from infection by the amoeba Entamoeba histolytica. The parasite most commonly is acquired in its encysted form in tainted food or drink. When it only infects the intestines it can cause dysentery, but it may also spread to other organs. Amebiasis is easily dealt with by drugs, but if untreated it can lead to abscesses of the liver, the lungs and, less frequently, the heart; rarely, it may even reach and damage the brain.

At least six forms of amoeba are parasitic in humans. Most important of these is Entamoeba histolytica, which causes amebiasis and dysentery. The diseases often occur in epidemics when raw sewage contaminates water supplies or when soil is fertilized with untreated human wastes.

Altitude Sickness

Altitude Sickness or Mountain Sickness, condition caused by reduced oxygen pressure at high altitudes, occurring in hikers as a consequence of relatively rapid ascent to high altitudes. Altitude sickness comprises a range of conditions, the most common being acute mountain sickness, which may be followed by two more critical conditions: high-altitude pulmonary edema or high-altitude cerebral edema.

The exact cause of altitude sickness is unknown, although researchers believe insufficient oxygen may impair the function of the sodium pumps located in cell walls. Without sufficient oxygen, the pumps allow sodium levels within the cell to rise. As a result, excess water enters the cells, causing cellular swelling, or edema, a condition that is especially dangerous in the lungs and around the brain.

The common symptoms of altitude sickness are headache, shortness of breath, sleep disturbances, and sometimes nausea. These symptoms, collectively called acute mountain sickness (AMS), appear in nearly 20 percent of people who ascend higher than about 2400 m (about 8000 ft) above sea level in less than one day. The headache is thought to be caused by cerebral edema. Shortness of breath is caused by a combination of reduced environmental-oxygen pressure and by the swelling and thickening of tissues through which oxygen must pass during respiration.

The symptoms associated with high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE) include those of AMS as well as additional, less common, symptoms. Both HAPE and HACE are more serious conditions than AMS, usually developing after one to four days above about 2700 m (about 9000 ft). Symptoms characterizing HAPE include strong coughing that produces a frothy, sometimes blood-tinged, sputum. The pressure of the blood in the lungs increases, and swollen cells and leaking capillaries allow fluids and red blood cells to enter the air sacs of the lungs, further impeding the delivery of oxygen. Symptoms characterizing HACE include staggering, confusion, and hallucination.