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allergen — a foreign substance to the body's immune system that may cause an allergic reaction.

allergies — disorders that involve an immune response in the body. Allergies are reactions to allergens such as plant pollen, other grasses and weeds, certain foods, rubber latex, insect bites, or certain drugs.

alpha-fetoprotein screening (AFP) — This blood test measures the levels of a substance called alpha-fetoprotein in the mother's blood. Abnormal levels can indicate a brain or spinal cord defect, the presence of twins, a miscalculated due date, or an increased risk of Down syndrome.

alveoli cells — tiny glands in the breast that produce milk.

amniocentesis — If necessary, this test is performed between 15 and 20 weeks of pregnancy and can indicate chromosomal abnormalities such as Down syndrome, or genetic disorders such as Tay Sachs disease, sickle cell disease, cystic fibrosis, and others. It also can detect the baby's sex and risk of spina bifida (a condition in which the brain or spine do not develop properly).

amniotic fluid — clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy. It is contained in the amniotic sac.

amputation - removal of part or all of a body part, except for organs in the body. It usually takes place during surgery in a hospital operating room. It is done because of injury to the body part or problems from diabetes, hardening of the arteries, or any other illness that impairs blood circulation. It is also done to prevent the spread of bone cancer. Many amputees are able to be fitted with an artificial limb.

anemia — when the amount of red blood cells or hemoglobin (the substance in the blood that carries oxygen to organs) becomes reduced, causing fatigue that can be severe.

aneurysm — a thin or weak spot in an artery that balloons out and can burst.

angina — a recurring pain or discomfort in the chest that happens when some part of the heart does not receive enough blood. It is a common symptom of coronary heart disease, which occurs when vessels that carry blood to the heart become narrowed and blocked due to atherosclerosis. Angina feels like a pressing or squeezing pain, usually in the chest under the breast bone, but sometimes in the shoulders, arms, neck, jaws, or back. Angina is usually is brought on by exertion, and relieved within a few minutes by resting or by taking prescribed angina medicine.

anorexia nervosa — an eating disorder caused by a person having a distorted body image and not consuming the appropriate calorie intake resulting in severe weight loss.

anovulation — absence of ovulation.

antibiotics — drugs used to fight many infections caused by bacteria. Some antibiotics are effective against only certain types of bacteria; others can effectively fight a wide range of bacteria. Antibiotics do not work against viral infections.

antibodies — proteins made by certain white blood cells in response to a foreign substance (antigen). Antibodies neutralize or destroy antigens.

antidepressants — a name for a category of medications used to treat depression.

antihistamines — drugs that are used to prevent or relieve the symptoms of hay fever and other allergies by preventing the action of a substance called histamine, which is produced by the body. Histamine can cause itching, sneezing, runny nose, watery eyes, and sometimes can make breathing difficult. Some of these drugs are also used to prevent motion sickness, nausea, vomiting, and dizziness. Since they may cause drowsiness as a side effect, some of them may be used to help people go to sleep.

antimetabolites — anticancer drugs that can stop or slow down biochemical reactions in cells.

anxiety disorder — serious medical illness that fills people's lives with anxiety and fear. Some anxiety disorders include panic disorder, obsessive-compulsive disorder, post-traumatic stress disorder, social phobia (or social anxiety disorder), specific phobias, and generalized anxiety disorder.

apnea — temporary interruption or cessation of breathing.

areola — the dark-colored skin on the breast that surrounds the nipple.

arteries — blood vessels that carry oxygen and blood to the heart, brain and other parts of the body.

artherosclerosis — disease when fatty deposits clog the walls of the arteries.

arthritis — swelling, redness, heat and pain of the joints. There are over 100 types of arthritis.

assisted reproductive technology — technology that involves procedures that handle a woman's eggs and a man's sperm to help infertile couples conceive a child.

asthma — a chronic disease of the lungs. Symptoms include cough, wheezing, a tight feeling in the chest, and trouble breathing.

atherosclerosis — a disease in which fatty material is deposited on the wall of the arteries. This fatty material causes the arteries to become narrow and it eventually restricts blood flow.

autoimmune — an immune response by the body against one of its own tissues, cells, or molecules.

autoimmune disease — disease caused by an immune response against foreign substances in the tissues of one's own body.