homeopathy Founded in the 19th century, a practice that is based on the concept that disease can be treated with minute doses of drugs thought capable of producing in healthy people the same symptoms as those of the disease being treated. This principle is similar to the concept behind exposure therapy for allergies, but the…
homeopath A person who practices homeopathy.
homeobox gene A gene containing a short DNA sequence of about 180 base pairs referred to as a homeobox. Homeobox genes encode proteins that bind and regulate the expression of DNA in multicellular organisms. Homeoboxes are present in the genomes of many organisms from fruit flies to humans, and they appear to determine when particular…
Holter monitor A type of portable heart monitor that is a small electrocardiogram (EKG) device worn in a pouch around the neck or waist. A Holter monitor keeps a record of the heart rhythm, typically over a 24-hour period, and the patient keeps a diary of activities and symptoms. The EKG recording is then correlated…
hole, macular See macular hole.
holandric inheritance Inheritance of genes on the Y chromosome. Because only males normally have Y chromosomes, Y-linked genes can only be transmitted from father to son.
Hodgkin’s lymphoma See Hodgkin’s disease.
Hodgkin’s disease A type of lymphoma, a cancer that develops in the lymph system, part of the body’s immune system. Because there is lymph tissue in many parts of the body, Hodgkin’s disease can start in almost any part of the body. The cancer can spread to almost any organ or tissue in the body,…
HLA Human leukocyte antigen, the major human histocompatibility system. HLA typing is done before transplantation to determine the degree of tissue compatibility between donor and recipient.
hive A raised, itchy area of skin that is often a sign of an allergic reaction. A hive can be rounded or flat topped but is always elevated above the surrounding skin. It reflects circumscribed edema (local swelling) of the skin. Hives are usually well circumscribed but may be coalescent, and they blanch with pressure….
HIV infection, primary The stage of infection by human immunodeficiency virus (HIV), in which detectable antibodies to HIV appear in the blood (seroconversion). It normally takes from several weeks to several months for antibodies to the virus to develop after HIV transmission. When antibodies to HIV appear in the blood, a person tests positive in…
HIV infection, acute The body’s initial reaction to infection by human immunodeficiency virus (HIV), a flu-like syndrome that occurs 2 to 4 weeks after a person contracts HIV. Symptoms include fever, sore throat, headache, skin rash, and swollen glands (lymphadenopathy). This syndrome precedes the development of detectable antibodies to HIV in the blood (seroconversion), which…