30 Mar

hyperplasia an increase in amount of tissue pro- duced by an increase in the number of cells. Hyper- plasia often accompanies the regeneration of a dam- aged organ. See hypertrophy. hyperploid referring to cells or individuals contain- ing one or more chromosomes or chromosome seg- ments in addition to the characteristic euploid num- ber. hyperprolinemia a hereditary disease in man aris- ing from a deficiency of the enzyme proline oxidase. hypersensitivity the characteristic of responding with clinical symptoms to allergens in amounts that are innocuous to most individuals. See allergy. hypertension an increased blood pressure. hyperthermophile a prokaryote that flourishes at very high temperatures. Some live under high pres- sures at great ocean depths and in the absence of sunshine.

They grow in tectonically active rift zones around volcanic vents. Some live at tempera- tures as high as 113°C! The group contains archae- ons like Archaeoglobus fulgidus and Methanococcus jannaschii and bacteria like Thermotoga maritima (see entry for each species). The species that have been placed closest to the trunk of the “universal tree of life” (q.v.) are all hyperthermophiles, and this sug- gests that the common ancestor of all prokaryotes may also have been a hyperthermophile. See extrem- ophiles, plate tectonics, undersea vent communities.

hypertrophy an increase in the size of a tissue or organ because of the increased volume of the com- ponent cells. See hyperplasia. hypervariable (hv) sites amino acid positions with- in the variable region of an immunoglobulin light chain or heavy chain, exhibiting great variation among antibodies of different specificity; these non- contiguous sites are brought together in the active site where antigen is bound (a paratope) by complex folding of the polypeptide chain. See immunoglob- ulin. hyphae branched or unbranched filaments that to- gether form the mycelium (q.v.) of a fungus.

A sin- gle filament is a hypha. hypo See fixing. hypochromic anemia See anemia. hypochromic shift reduction in the absorption of ultraviolet light as complementary single strands of DNA unite to form duplexes. See hyperchromic shift. hypodontia the congenital absence of teeth. hypoglycemia a decrease in sugar content of the blood serum. hypomorph any allele that permits a subnormal expression of the normal phenotype. For example, a mutated allele may encode an enzyme that is unsta- ble. However, enough functional enzyme may be generated so that the reaction proceeds, but slowly.

Since the genetic block is incomplete, a hypomor- phic allele is sometimes called “leaky.” hypophosphatasia a hereditary disease in humans arising from a deficiency of the enzyme alkaline phosphatase. hypophosphatemia a decreased concentration of inorganic phosphate in the blood serum. hypophysis the pituitary gland. hypoplasia an arrested development of an organ or part. The opposite of hyperplasia (q.v.). hypoploid referring to cells or individuals contain- ing one or more fewer chromosomes or chromosome segments than the characteristic euploid number. hypostatic gene See epistasis. hypothalamus the floor and sides of the vertebrate brain just behind the attachment of the cerebral hemispheres. The hypothalamus controls the secre- tion of a variety of releasing hormones.

These are transported down a closed portal system to the pitu- itary gland. Here releasing hormones bind to re- ceptors on cells in the anterior lobe. These cells then secrete hormones into the circulatory system that eventually bind to receptors in specific tissues.

Hypothalamic releasing hormones include: prolac- tin-releasing factor, somatostatin, somatocrinin, thy- rotropin-releasing hormone, and gonadotropin- releasing hormone. See human growth hormone (hGH). hypothyroidism a diminished production of thy- roid hormone. hypoxanthine 6-hydroxypurine. See purine. hypoxanthine-guanine-phosphoribosyl transfer- ase the enzyme that catalyzes the transfer of the phosphoribosyl moiety of 5-phosphoribosyl-1-pyro- phosphate to the 9 position of hypoxanthine and guanine to form inosine monophosphate and guano- sine monophosphate. Abbreviated HPRT or HG- PRT. The Lesch-Nyhan syndrome (q.v.) is caused by deficiency of HPRT. See Appendix C, 1987, Kuehn et al.; HAT medium.


Hyracotherium the genus that contains the earliest ancestors of the horse. Adults of the smallest species in this genus were only about 10 inches high at the shoulders. These fossils were first described in Eocene deposits in Europe. In the North American Eocene rocks, where fossils of horses were more abundant, the genus was named Eohippus (dawn horse). When it later became clear that the two gen- era represented the same animals, Hyracotherium was chosen as the correct scientific name, since it had been coined earlier (1840, rather than 1876). See Linnean system of binomial nomenclature.

i the regulator gene of the lactose operon in E. coli. See regulator genes. I iodine. I1, I2, I3, etc. the first, second, third, etc., genera- tions obtained by inbreeding. IA, IB, i the allelic genes responsible for the ABO blood group system. See A, B antigens. IAA indole acetic acid (q.v.).

Ia antigens alloantigens encoded by the Ia region of the mouse major histocompatibility complex (H-2). They are defined by serological methods and are found predominantly (but not exclusively) on B lymphocytes and macrophages. ICM inner cell mass (q.v.). icosahedron a regular geometric polyhedron com- posed of 20 equilateral triangular faces with 12 cor- ners. The capsids of many spherical eukaryotic vi- ruses and bacteriophages are icosahedral.

See adenovirus, enveloped viruses, herpesvirus, polio vi- rus, Q beta (Qβ) phage, Shope papillomavirus, virus. ICSH interstitial cell-stimulating hormone. Identi- cal to LH (q.v.). icterus jaundice (q.v.). identical twins See twins. idiocy the most severe degree of mental retarda- tion. An idiot reaches an intelligence level below that of a two-year-old child. idiogram a diagrammatic representation of the karyotype (q.v.) of an organism. idiotypes antigenic determinants characteristic of a particular variable domain of a specific immuno- globulin or T cell receptor molecule. The idiotype is a unique attribute of a particular antibody from a specific individual. Contrast with allotypes, isotypes. idling reaction production of ppGpp and pppGpp by ribosomes when an uncharged tRNA is present in the A site.

See translation. IF initiation factor (q.v.). IFNs interferons (q.v.). Ig immunoglobulin (q.v.). IgA human immunoglobulin A, found as a 160- kilodalton monomer or as a 320-kilodalton dimer in mucus and secretory fluids and on the surface of cell membranes. IgD human immunoglobulin D, found as a 185- kilodalton monomer on the surface of lymphocytes. IgE human immunoglobulin E, found as a 200- kilodalton monomer and involved in allergic reac- tions. It forms a complex with antigen and then binds to the surface of mast cells, triggering the re- lease of histamine. Igf 2 insulin growth factor 2. See H19.

IgG human immunoglobulin G, found as a 150- kilodalton monomer, which is the predominant mol- ecule involved in secondary immune responses. It fixes complement and is the only immunoglobulin that crosses the placenta. See Appendix C, 1969, Edelman et al.; immune response. IgM human immunoglobulin M, found as a 900- kilodalton pentamer that is the predominant mole- cule involved in the primary immune response. It fixes serum complement and agglutinates effec- tively. ile isoleucine. See amino acid. imaginal discs inverted thickenings of epidermis containing mesodermal cells found in a holometabo- lous insect. During the pupal stage, the imaginal discs give rise to the adult organs, and most larval structures are destroyed.

See Appendix C, 1973, Gar- cia-Bellido et al.; 1975, Morata and Lawrence; com- partmentalization, in vivo culturing of imaginal discs. imino forms of nucleotides See tautomeric shifts. immediate hypersensitivity a type of hypersensi- tivity reaction that is mediated by antibodies and that occurs within minutes after exposure to the al- lergen or antigen in a previously sensitized individ- ual. Compare with delayed hypersensitivity.

immortalizing genes genes carried by oncogenic viruses that confer upon cultured mammalian cells the ability to divide and grow indefinitely, thereby overcoming the Hayflick limit (q.v.). immune competent cell a cell capable of produc- ing antibody in response to an antigenic stimulus. immune decoy protein See sporozoite. immune globulins See antibody. immune response the physiological response(s) stemming from activation of the immune system by antigens, including beneficial immunity to pathogenic microorganisms, as well as detrimental autoimmunity to self-antigens, allergies, and graft rejection.

The cells mainly involved in an immune response are T and B lymphocytes and macrophages. T cells produce lym- phokines (q.v.) that influence the activities of other host cells, whereas B cells mature to produce immu- noglobulins (q.v.) or antibodies that react with an- tigens. Macrophages “process” the antigen into im- munogenic units that stimulate B lymphocytes to differentiate into antibody-secreting plasma cells, and stimulate T cells to release lymphokines. Com- plement (q.v.) is a group of normal serum proteins that can aid immunity by becoming activated as a consequence of antigen-antibody interactions.

The first contact with an antigen “sensitizes” the animal and results in a primary immune response. Subse- quent contact of the sensitized animal with that same antigen results in a more rapid and elevated reaction, called the secondary immune response (also known as the “booster response” or the “anamnestic reaction”), which is most easily demonstrated by monitoring the level of circulating antibodies in the serum. The immune response can be transferred from a sensitized to an unsensitized animal via se- rum or cells. It is highly specific for the inciting anti- gen, and is normally directed only against foreign substances.

See adenosine deaminase deficiency. immune response (Ir) gene any gene that deter- mines the ability of lymphocytes to mount an im- mune response to specific antigens. In the major his- tocompatibility complex of the mouse (the H-2 complex), the I region contains Ir genes and also codes for Ia (immune-associated) antigens found on B cells and on some T cells and macrophages. In hu- mans, the HLA D (DR) region is the homolog of the mouse H-2 I region.

See Appendix C, 1948, Snell; 1963, Levine el al.; 1972, Benacerraf and McDevitt. immune system the organs (e.g., thymus, lymph nodes, spleen), tissues (e.g., hematopoietic tissue of bone marrow, mucosal and cutaneous lymphoid tissues), cells (e.g., thymocytes, blood and tissue lymphocytes, macrophages), and molecules (e.g., complement, immunoglobulins, lymphokines) re- sponsible for immunity (protection against foreign substances). immunity 1. the state of being refractive to a spe- cific disease, mediated by the immune system (T and B lymphocytes and their products—lymphokines and immunoglobulins, respectively).

Active immu- nity develops when an individual makes an immune response to an antigen; passive immunity is acquired by receiving antibodies or immune cells from an- other individual. 2. the ability of a prophage to in- hibit another phage of the same type from infecting a lysogenized cell (phage immunity). 3. the ability of a plasmid to inhibit the establishment of another plasmid of the same type in that cell. 4. the ability of some transposons to prevent others of the same type from transposing to the same DNA molecule (transposon immunity). 5. phage-resistant bacteria are usually “immune” to specific phages because they lack the cell-surface receptors that define the host range of that phage.

See innate immunity. immunity substance a cytoplasmic factor pro- duced in lysogenic bacteria that prevents them from being infected by bacteriophages of the same type as their prophages and also prevents the vegetative replication of said prophages. immunization administration of an antigen for the purpose of stimulating an immune response to it. Also known as inoculation or vaccination. immunochemical assay any technique that uses antigen-antibody reactions to detect the location of or to determine the relative amounts of specific anti- bodies or antigenic substances. See enzyme-linked immunosorbent assay, immunofluorescence assay. immunocompetent (immune competent) cell a cell capable of carrying out its immune function when given the proper stimulus.


immunodominance within a complex immuno- genic molecule, the ability of a specific component (1) to elicit the highest titer of antibodies during an immune response, or (2) to bind more antibodies from a given polyvalent antiserum than any other component of that same molecule. For example, in a glycoprotein antigen, a specific monosaccharide may be the most highly antigenic component of the entire molecule and therefore exhibits immunodom- inance over other components of the same molecule. immunoelectrophoresis a technique that first separates a collection of different proteins by elec- trophoresis through a gel and then reacts them with a specific antiserum to generate a pattern of precipi- tin arcs. The proteins can thus be identified by their electrophoretic mobilities and their antigenic prop- erties. See Appendix C, 1955, Grabar and Williams. immunofluorescence assay a visual examination of the presence and the distribution of particular an- tigens on or in cells and tissues using antibodies that have been coupled with fluorescent molecules such as rhodamine and fluorescein.

In the direct method, the fluorescent probe combines directly to the anti- gen of interest. In the indirect method, two antibodies are used in sequence. The first is the one specifically against the antigen under study. Subsequently, the tissue is incubated with a second antibody, prepared against the first antibody. The second antibody has been conjugated previously with a fluorescent dye, which renders the complex visible. The indirect method is often preferred because, if one wants to localize more than one antigen, only one fluores- cently labeled antigen need be used, provided the first antibody in each case is from the same species of animal. The second fluorescent antibody is gener- ally commercially available.

See Appendix C, 1941, Coons et al. immunogen a substance that causes an immune response. Foreign proteins and glycoproteins gener- ally make the most potent immunogens. See antigen. immunogene any genic locus affecting an immu- nological characteristic; examples: immune response genes, immunoglobulin genes, genes of the major histocompatibility complex (all of which see). immunogenetics studies using a combination of immunologic and genetic techniques, as in the inves- tigation of genetic characters detectable only by im- mune reactions.

See Appendix C, 1948, Snell; 1963, Levine et al.; 1972, Benacerraf and McDevitt. immunogenic capable of stimulating an immune response. immunoglobulin an antibody secreted by mature lymphoid cells called plasma cells. Immunoglobulins are Y-shaped, tetrameric molecules consisting of two relatively long polypeptide chains called heavy (H) chains and two shorter polypeptide chains called light (L) chains (see illustration on page 227). Each arm of the Y-shaped structure has specific antigen- binding properties and is referred to as an antigen- binding fragment (Fab). The tail of the Y structure is a crystallizable fragment (Fc). Five H chain classes of immunoglobulin are based upon their antigenic structures.

Immunoglobulin class G (IgG) is the most common in serum and is associated with im- munological “memory”; class IgM is the earliest to appear upon initial exposure to an antigen. Class IgA can be secreted across epithelial tissues and seems to be associated with resistance to infectious diseases of the respiratory and digestive tracts. The antibodies associated with immunological allergies belong to class IgE. Not much is known about the functions of IgD. Antibodies of classes IgG, IgD, and IgE have molecular weights ranging from 150,000 to 200,000 daltons (7S); serum IgA is a 7S monomer, but secre- tory IgA is a dimer (11.4S); IgM is a pentamer (19S; 900,000 daltons) of five 7S-like monomers. In the case of IgG, each heavy chain consists of four “domains” of roughly equal size.

The variable (VH) domain at the amino (N-terminus) end con- tains different amino acid sequences from one im- munoglobulin to another, even within the same H chain class. The other three domains have many re- gions of homology that suggest a common origin by gene duplication and diversification by mutation. These “constant” domains (CH1, CH2, CH3) are es- sentially invariate within a given H chain class.

An L chain is about half as long as an H chain. Its amino end has a variable region (VL); its carboxyl end has a constant region (CL). An Fab fragment consists of an L chain and an Fd segment of an H chain (VH + CH1). Within a tetrameric immunoglobulin mole- cule, the two L chains are identical and the two H chains are identical. The Fc fragment consists of carboxy-terminal halves of two H chains (CH2 + CH3). The region between CH1 and CH2 is linear rather than globular, and is called the hinge region. Crystallographic studies of human IgG show that the oligosaccharide chains (OC) that are attached to the CH2 regions provide surfaces that bind these re- gions to each other and to the Fab units. Each ma- ture antibody-synthesizing plasma cell produces a single species of immunoglobulin, all of which con-

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