Allogenic In transplantation biology, denoting individuals (or tissues) that are of the same species but antigenically distinct.
Allograft Tissue graft between individual of the same species but of different genotype; also called homograft.
Alopecia A disease in which hair falls out
Alpha Particle Helium atom stripped of its two electrons, that is, helium atom with only two protons and two neutrons and has positive charge. When alpha particles are ejected from radioactive material, they interact with all matter in their path and produce large number of ions before gaining two electrons and becoming neutral helium. Alpha particles are unable to penetrate outer layer of human skin, but if taken internally they can cause massive damage of the surrounding tissue. For this reason alpha emitters are never used in nuclear medicine.
Alpha-amylase A hydrolytic enzyme that breaks polysaccharides. It hydrolyses 1,4-α-glycosidic linkage in polysaccharides.
Alpha-fetoprotein A plasma protein produced by the fetal liver and gastrointestinal tract; blood levels decline markedly by the age of one year but are again elevated in many cancerous condition and in some benign liver diseases such as cirrhosis and viral hepatitis.
Alpha-helix A secondary structure occurring in many proteins; it is a right-handed helix with 3.6 amino acid residues per turn stabilized by hydrogen bonds between the imino hydrogen of each peptide bond and the carbonyl oxygen of the peptide bond four residues further along the polypeptide chain.



Amines organic compounds containing nitrogen.
Amino-glycoside A cyclic chemical compound containing amino sugars. Amino-glycoside bacterial antibiotics inhibit bacterial protein synthesis.
Anabolic Pertaining to constructive synthetic metabolic processes by which organisms convert substances into other chemical structures.
Analogue A chemical compound with a structure similar that of another, but differing from it in respect to a certain component.
Androgens The masculinizing hormones such as androsterone and testosterone produced by testes.
Aneuploid cells Cells with number of chromosomes that deviate from haploid number (a single set of nonhomologous chromosomes in human is 23).
Aneuploidy Any deviation from an exact multiple of the haploid number (a single set of nonhomologous chromosomes in human is 23) of chromosomes, whether fewer or more.
Angiofibroma Tumors of blood or lymph vessels containing fibrous tissue.
Angiogenesis The formation of new blood vessels.
Angiogenic Factors Factors that stimulate the formation of new blood vessels.
Angiography Radiographic visualization of blood vessels after introduction of a contrast material; used as a diagnostic aid.
Angiotensin II An octapeptide hormone present in blood produced by the enzymatic activation of the inactive decapeptide angiotensin I. It is a powerful vasopressor and a stimulator of aldosterone secretion by the adrenal cortex.
Anion An ion is an atom or molecule that has acquired a charge by either gaining or losing electrons; one with extra electrons has a net negative charge and is called an anion.
Ankylosing spondylitis A chronic progressive form of arthritis accompanied by inflammation and eventual immobility and consolidation of a number of joints especially the spine.
Annealing Heating at high temperature and then controlled slow cooling.
Anorexia The uncontrolled loss of appetite for food.
Anthracyclines: Organic compounds that have a tetrahydronaphthacenedione ring structure attached by a glycosidic linkage to the amino sugar daunosamine.
Antiblastic Retarding growth or multiplication.
Antibodies Specific proteins synthesized and secreted by B-lymphocytes of immune system in response to antigen. These are also called immunoglobulins, which have a specific amino acid sequence by virtue of which they interact only with the antigen that induced their synthesis and destroys it.
Antigen Any foreign agent that can elicit immune response, i.e., stimulates the production of specific proteins by specific cells of immune system. These proteins in turn destroy the foreign agent.
Antihormone Any chemical that binds to the hormone receptor but does not have the activity of the hormone.
Anti-idiotypic Pertaining to an antibody directed against an idiotypic determinant of another antibody. Idiotypes distinguish a clone of immunoglobulin-producing cells from other clones.
Antimetabolite A substance bearing a close structural resemblance to one required for normal physiological functioning. It exerts its effect by interfering with the utilization of essential metabolite.
Antisense In molecular genetics, referring to the strand of a double-stranded DNA that is complementary to the sense strand, the strand of DNA which is used during transcription to make mRNA.

Antisera: Sera that contain antibodies; it may be obtained from an animal that has been immunized either by injection of an antigen into the body or by infection with microorganisms containing the antigen.