Antibiotic resistance - the ability of a microorganism to produce a protein that disables an antibiotic or prevents transport of the antibiotic into the cell.

Antibody - an immunological protein (called an immunoglobulin, Ig) produced by certain white blood cells lymphocytes) of the immune system of an organism in response to a contact with a foreign substance (antigen). Such an immunological protein has the ability of specifically binding with the foreign substance and rendering it harmless. The basic immunoglobulin molecule consists of two identical heavy and two identical light chains

Antibody class - the class to which an antibody belongs, depending on the type of heavy chain present. In mammals, there are five classes of antibodies: IgA, IgD, IgE, IgG, and IgM

Antibody structure - antibodies have a well-defined structure. Each antibody has two identical "light" chains and two identical "heavy" chains. Each chain comprises a constant region, i.e., a region that is the same between antibodies of the same class and sub class, and a variable region that differs between. The antigen-binding region or binding site complementariry determining region is in the variable region. The antibody can be cut by proteases into several fragments, known as Fab, Fab', and Fc

Antibody mediated (humoral) immune response - the synthesis of antibodies by B cells in response to an encounter of the cells of the immune system with a foreign Immunogen

Anticlinal - the plane of cell wall orientation or cell division perpendicular (at right angles) to the surface of the organ

Anticoding strand - the strand of the DNA double helix that is actually transcribed. Also known as the antisense or template strand

Anticodon - a triplet of nucleotides in a tRNA molecule that pairs with a complementary triplet of nucleotides, or codon, in an mRNA molecule during translation

Antigen: immunogen - a compound that elicits an immune response by stimulating the production of antibodies. The antigen, usually a protein, when introduced into a vertebrate organism is bound by the antibody or a T cell receptor

Antigenic determinant - a surface feature of a microorganism or macromolecule, such as a glycoprotein, that elicits an immune response

Antigenic switching - the altering of a microorganism's surface antigens through genetic re-arrangement, to elude detection by the host's immune system

Antihaemophilic globulin - blood globulin that reduces the clotting time of haemophilic blood

Anti-idiotype antibodies - antibodies which recognise the binding sites of other antibodies. Their binding sites are complementary to the binding sites of another immunoglobulin. When an animal becomes immune to something, it not only acquires antibodies against that something, it also acquires antibodies against those antibodies. This forms a network of antibodies which can all bind to each other to various degrees, helping to regulate the immune response. Some allergic responses are in part due to the breakdown of this sort of regulation

Antimicrobial agent - any chemical or biological agent that harms the growth of micro-organisms

Antinutrients - compounds that inhibit normal uptake of nutrients

Anti oncogene - a gene whose product prevents the normal growth of tissue. cf recessive oncogene

Antioxidant - compound that slows the rate of oxidation reactions

Antioxidant solution - pre-treatment solution (e.g., Vitamin C; citric acid) that retards senescence and browning of tissue. It is employed to incubate explants prior to surface sterilisation

Antiparallel orientation - the normal arrangement of the two strands of a DNA molecule, and of other nucleic-acid duplexes (DNA-RNA, RNA-RNA), in which the two strands are oriented in opposite directions so that the 5' -phosphate end of one strand is aligned with the 3'-hydroxyl end of the complementary strand

Antisense DNA - 1. the strand of chromosomal DNA that is transcribed
2. a DNA sequence that is complementary to all or part of an mRNA molecule

Antisense gene - a gene that produces a transcript (mRNA) that is complementary to the pre-mRNA or mRNA of a normal gene (usually constructed by inverting the coding region relative to the promoter)

Antisense RNA - an RNA sequence that is complementary to all or part of a functional mRNA molecule, to which it bind , blocking its translation. See RNA

Antisense therapy - the in vivo treatment of a genetic disease by blocking translation of a protein with a DNA or an RNA sequence that is complementary to a specific mRNA

Antiseptic - any substance that kills or inhibits the growth of disease causing micro-organism (a micro-organism capable of causing sepsis), but is essentially non-toxic to cells of the body

Antiserum - the fluid portion of the blood 2 of an animal (after coagulation of the blood), containing antibodies

Anti-terminator - a type of protein which enables RNA polymerase to ignore certain transcriptional stop or termination signals and read through them to produce longer mRNA transcripts

Antitranspirant - a compound designed to reduce transpiration when sprayed or painted on leaves of newly transplanted trees, shrubs or vines, or used as a dip for cuttings in lieu of misting; may interfere with photosynthesis and respiration if the coating is too thick or unbroken

Apex - the tip, point or angular summit. The tip of a leaf; the portion of a root or shoot containing apical and primary meristems. Usually used to designate the apical tip of the meristem

Apical cell - a meristematic initial in the apical meristem of shoots or roots of plants. As this cell divides, new tissues are formed