T-4 cell A T cell that expresses the CD4 transmembrane glycoprotein (CD4+ T cell). T-4 cells are active in the body’s immune response, helping to turn on this system when it is challenged by an infection or by foreign matter in the body. HIV attacks T-4 cells, knocking out the body’s ability to defend itself…
T-4 count A test that counts the number of T-4 cells in the blood, for example, to assess the immune status of a patient with HIV. Of the various ways to read a T-4 count test, the best indicator of health may be the absolute T-4 count, the actual number of T-cells per unit volume…
thalamotomy A procedure that is performed via stereotactic surgery and is designed to destroy part of the thalamus in order to relieve intractable pain, seizures, or involuntary movements, as in Parkinson’s disease.
thalamus A large ovoid mass within the midbrain (the interbrain, connected to the cerebral hemisphere) that forms part of the walls of the third ventricle in the brain. The thalamus contains a number of distinct groups of cells, or nuclei, that function as relay centers for sensory and other impulses between the body and the…
thalassemia A group of genetic disorders that involve underproduction of hemoglobin, the indispensable molecule in red blood cells that transports oxygen and carbon dioxide. All forms of hemoglobin are made up of two molecules: heme and globin. The globin part of hemoglobin is made up of four polypeptide chains. In normal adult hemoglobin (Hb A),…
thalassemia, alpha A form of thalassemia that involves the hemoglobin alpha chain. If a fetus inherits two genes for alpha thalassemia, one from each parent, the disorder is lethal before birth: No alpha chains can be made, and without alpha chains, there can be no fetal hemoglobin. If the fetus inherits only one alpha thalassemia…
thalassemia, beta See thalassemia major.
thalassemia major The most serious form of beta thalassemia, in which there is a mutation in both of the beta globin chains of hemoglobin. This leads to underproduction or absence of beta chains, underproduction of hemoglobin, and profound anemia. Children with thalassemia major seem entirely normal at birth because at birth they still have predominantly…
thalassemia minor Also called thalassemia trait, the carrier state for thalassemia major. People who are carriers have just one beta thalassemia gene and are essentially normal, although they can transmit the gene to their offspring. See also thalassemia.
thalidomide A drug (brand name: Thalomid) that was used in the 1950s and early 1960s to treat morning sickness in pregnancy. The use of thalidomide had tragic results: When taken during pregnancy, it can cause a syndrome of congenital malformations in the developing fetus. Thalidomide was then not approved for marketing in the US and…
thalidomide baby An infant who is affected by prenatal exposure to the drug thalidomide. In 1998, thalidomide again became available to treat certain conditions, and there is concern that babies may be born with the thalidomide syndrome as they were in the 1950s and early 1960s.
thalidomide syndrome A congenital malformation syndrome that is caused by maternal exposure to thalidomide. Although thalidomide syndrome is epitomized by the flipper-like limbs (phocomelia), thalidomide causes a wide range of birth defects, including absence of ears with deafness; defects of the muscles of the eye and of the face; absence or hypoplasia (underdevelopment) of the…