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The Ebola virus comprises a group of pathogenic agents that cause severe and deadly hemorrhagic fevers in humans and other primates. During 2014 and 2015, an Ebola virus epidemic in West Africa—predominantly, the countries of Guinea, Liberia, and…

A family of adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or…

One of the major plant hormones that plays an important role in plant growth and development. Abscisic acid (ABA) is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. Abscisic acid is…

The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin, and in…

A naturally occurring quaternary ammonium cation ester, with the formula CH3(O)COC2H4N(CH3)3+, which plays a prominent role in nervous system function as a neurotransmitter. Acetylcholine (Fig. 1) is an organic compound released from certain…

An order of monocotyledonous angiosperms composed of a single genus, Acorus (sweet flag or sweet calamus), with two species widespread in the Northern Hemisphere. Formerly, Acorus was included in the aroid family, Araceae, but several lines of…

An autonomous undersea vehicle that can be launched from submarines, surface ships, or aircraft to attack enemy submarines and surface ships. An acoustic torpedo is a sophisticated weapon. Its main components are a guidance and control system, a…

A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

An induced state in which antigens originally regarded as foreign become regarded as self by the immune system. Tolerance can be induced (tolerization) in all of the cells of the immune system, including T cells (also known as T lymphocytes), the…

A genus of gram-negative, immotile and nonspore-forming, oval to rod-shaped, often pleomorphic bacteria which occur as parasites or pathogens in mammals (including humans), birds, and reptiles. They are facultatively aerobic, capable of fermenting…

A heterogeneous collection of bacteria with diverse ecologies and physiologies. The initial definition was based on their branching filamentous cellular morphology during a stage of the growth cycle. The name actinomycete means “ray fungus,” and for…

A characteristic of an organism that makes it fit for its environment or for its particular way of life. Adaptation is a key biological process in organisms and can take many forms. Typically, adaptation refers to two mechanisms: (1) an adjustment…

A type of defective virus particle in which part of the genetic material of papovavirus SV40 is encased within an adenovirus protein coat (capsid). Human adenoviruses require human cells for their propagation; however, papovavirus SV40 can serve as…

A family of double-stranded DNA viruses, collectively known as adenoviruses, which are icosahedrally symmetric, usually found in the respiratory tract of the host species, and often associated with respiratory diseases. Members of the Adenoviridae…

A complex endocrine organ in proximity to the kidney. Adrenal gland tissue is present in all vertebrates from cyclostomes to placental mammals. The adrenal consists of two functionally distinct tissues: steroidogenic cells of mesodermal origin and…

A bacterial genus in the family Vibrionaceae comprising oxidase-positive, facultatively anaerobic, monotrichously flagellated gram-negative rods. The mesophilic species are A. hydrophila, A. caviae, and A. sobria; the psychrophilic one is A.…

A reaction in which suspended particles are aggregated or clumped. It occurs upon the admixture of another type of particle, a change in the composition of the suspending fluid, or the addition of a soluble agent that acts as a bridge between two or…

A substance that will cause a clumping of particles such as bacteria or erythrocytes. Of major importance are the specific or immune agglutinins, which are antibodies that will agglutinate bacteria containing the corresponding antigens on their…

An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental…

A plasma protein produced by the liver that maintains fluid balance in the blood and transports fatty acids in the plasma and interstitial fluid. It is the most abundant protein in human serum, and one of the first discovered and earliest studied…

A fluid-filled sac- or sausagelike, extraembryonic membrane lying between the outer chorion and the inner amnion and yolk sac of the embryos of reptiles, birds, and mammals. The allantois eventually fills up the space of the extraembryonic coelom in…

Any of a number of alternative forms of a gene. Allele is a contraction of allelomorph, a term that William Bateson used to designate one of the alternative forms of a unit showing mendelian segregation. New alleles arise from existing ones by…

Altered reactivity in humans and other animals to allergens (substances foreign to the body that trigger an allergic response) induced by exposure through injection, inhalation, ingestion, or skin contact. An allergy (or allergic reaction; Fig. 1)…

The study of changes in the characteristics of organisms with body size. Characteristics such as body parts or timing of reproductive events do not necessarily change in direct proportion to body size, and the ways in which they change relative to…

A glycoprotein that is normally present in significant amounts only in the serum of the fetus. It is produced in the yolk sac, the liver, and other tissues of the gastrointestinal tract. Its role is unknown, but alpha fetoprotein may function as a…

Any of the organic compounds possessing one or more basic amino groups and one or more acidic carboxyl groups. Of the more than 80 amino acids that have been found in living organisms, approximately 20 serve as the building blocks for proteins.…

A thin, cellular, extraembryonic membrane forming a closed sac which surrounds the embryo in all reptiles, birds, and mammals and is present only in these forms; hence the collective term amniotes is applied to these animals. The amnion contains a…

A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia.…

An enzyme that is essential for maintenance of cellular energy balance. All living cells carry out many energy-requiring processes, including growth and division, movement, and uptake of nutrients from their environment. To achieve this, they…

A stimulating drug that affects the brain and the body. Amphetamine is a stimulant of the central nervous system. Chemically, amphetamine is a racemic mixture of the l and d isomers of α-methyl-β-phenethylamine (see illustration). The l isomer has…

A disorder characterized by the accumulation of an unusual extracellular fibrous protein (amyloid) in the connective tissue of the body. The deposition of amyloid may be widespread, involving major organs and leading to serious clinical…

A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the…

A generalized or localized tissue reaction occurring within minutes of an antigen-antibody reaction. Similar reactions elicited by nonimmunologic mechanisms are termed anaphylactoid reactions. The term anaphylaxis was introduced by C. R. Richet to…

Synthetic manufactured drugs derived from the male sex hormone testosterone. Androgenic-anabolic steroids, often termed simply as anabolic steroids, are artificially produced testosterone variants. They can provide a number of medical benefits when…

A class of steroid hormones produced in the testes and adrenal cortex that act to regulate masculine secondary sexual characteristics. The major naturally occurring androgens, such as testosterone, are C-19 steroids (that is, they each contain 19…

A medical condition resulting from a reduction in the total quantity of hemoglobin or of red blood cells in the circulation. Anemia is a disorder marked by significant decreases in hemoglobin concentration and in the number of circulating red blood…

The condition of exhibiting no immune response to an antigen. Anergy is a self-tolerance mechanism in which the immune system shows unresponsiveness following an encounter with receptor-recognized antigens. Typically, lymphocytes (the B and T cells…

The origin and development of blood vessels. Angiogenesis is the process through which new blood vessels are formed. Blood vessels (Fig. 1) are composed of two basic cell types: vascular endothelial cells and periendothelial cells (including…

The application of genetic principles for improving heredity for economically important traits in domestic animals. The science of animal breeding is based on fundamental genetic concepts. Through controlled mating and selection, which can involve…

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's…

The increase in size or weight of an animal. A number of processes are involved in growth. These include hyperplasia, or increase in cell number due to cell proliferation (cell division or mitosis) and recruitment from stem cell populations;…

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain…

The formation of new individuals, which may occur by asexual or sexual means. All animals must reproduce asexually or sexually to create new individuals of their species. In asexual reproduction, a single parent can produce offspring that are…

The symmetrical disposition of organs and other constituent parts of the bodies of living animals with respect to imaginary axes. Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests.…

The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

An acute, infectious worldwide zoonotic disease (transmissible to humans) caused by the spore-forming bacterium Bacillus anthracis to which most animals, especially grazing herbivores, are susceptible. Anthrax is a serious, though rare, infectious…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

The observation, measurement, and explanation of human variability in time and space. Anthropology is a science concerned with the interrelations of the biological, cultural, geographical, and historical aspects of humankind, including modern human…

The capability that a bacterium acquires to counteract an inhibitory chemical molecule or compound that was formerly effective in killing it or preventing its growth. Antibiotic resistance in bacteria (Fig. 1) is a hot topic in the media and in…

Glycoprotein or protein molecules synthesized by polar and north temperate fishes to enable them to survive in freezing seawater. Similar antifreezes are found in some insects, but relatively little is known about their structure and…

A reaction that occurs when an antigen combines with a corresponding antibody to produce an immune complex. A substance that induces the immune system to form a corresponding antibody is called an immunogen. All immunogens are also antigens because…

A chemical substance used to destroy or prevent the growth of infectious microorganisms on or in a human or other animal body. An antiseptic is an antimicrobial compound or drug preparation that inhibits the spread and development of harmful…

An antibody that will combine with and generally neutralize a particular toxin. When the manifestations of a disease are caused primarily by a microbial toxin, the corresponding antitoxin, if available in time, may have a pronounced prophylactic or…

The main vessel of the systemic arterial circulation arising from the heart. The aorta (see illustration) is the main artery leaving the heart in all vertebrates and some invertebrates. In mammals, reptiles, and birds, which possess two heart…

One of the cellular aggregations traditionally known as chemoreceptors which are generally found adjacent to the aortic arch and pulmonary artery and which often are adjacent to the origin of the subclavian arteries (see illustration). The term is…

Programmed cell death triggered by extracellular signals or genetically determined events and carried out by physiological processes within the cell. Apoptosis is one of the major forms of cell death in multicellular organisms (necrosis is the other…

A narrow, elongated tube closed at one end, extending from the cecum (a blind pocket off the first part of the large intestine). The appendix (see illustration), also known as the vermiform appendix, is found in only a few mammals. The size and…

A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota (also termed Eukarya or Eucarya) [Fig. 1]. The members of…