Zygote

the cell produced by the fusion of male and female germ cells (GAMETES) during the initial stage of FERTILIZATION. The zygote is a DIPLOID cell, formed by the fusion of the haploid male gamete and the haploid female gamete.

Taken from Dictionary of Science

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A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"), or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and a sperm cell from a male — which combine to form the single diploid cell. Such zygotes contain DNA derived from both the mother and the father, and this provides all the genetic information necessary to form a new individual. The term zygote is also used more loosely to refer to the group of cells formed by the first few cell divisions, although this is properly referred to as a morula.

In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cells without the zygote actually increasing in size. This cell division is mitotic, and is known as cleavage. All mammals go through the zygote stage of life. Zygotes eventually develop into an embryo, and then a fetus. A human zygote exists for about four days, and becomes a blastocyst on the fifth day.

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T-cell

a type of white blood cell (LYMPHOCYTE) that forms in the bone marrow and moves to the thymus gland, situated in the thorax. There are a whole variety of T-cells involved in recognizing particular foreign bodies (ANTIGENS), and they are important in fighting viral infections and destroying bacteria that have penetrated the cells of the body.

Taken from Dictionary of Science

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T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocyte types, such as B cells and natural killer cells by the presence of a special receptor on their cell surface called T cell receptors (TCR). The abbreviation T, in T cell, stands for thymus, since this is the principal organ responsible for the T cell's maturation. Several different subsets of T cells have been discovered, each with a distinct function.

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Bacterium

(plural bacteria) a MICRO-ORGANISM (one that cannot be seen with the naked eye but only with the aid of a MICROSCOPE), which usually has a body made of only one cell (unicellular). Bacteria are believed by many scientists to be the first organisms to have existed on Earth and they have always been extremely important in all life processes. Bacteria occur in water, air, soil and rotting plant or animal debris, and even in extremely inhospitable environments such as hot springs full of chemicals. They are vitally important in the breakdown and decomposition of ORGANIC material--without bacteria, nothing would rot away. Most bacteria need oxygen in order to live and are called aerobic, but others do not and are termed anaerobic. The surrounding layer around a bacterial cell is called the cell wall. Two types of bacteria, which each have a different cell wall structure (Gram positive or Gram negative) are identified by a test known as Gram's stain. Some bacteria have a protective, slimy outer layer called a capsule, and others may have hairs called filaments which cause movement. Bacteria occur in a number of different shapes and forms which help to identify them. These are spiral (plural spirilli, singular, spirillus), spherical or round (plural cocci, singular coccus), rod-like (plural bacilli, singular, bacillus), comma-shaped (vibrio) and corkscrew-shaped (spirochaetae). Bacteria usually reproduce by asexual reproduction and a few are responsible for extremely serious diseases in plants, animals and man, e.g. typhoid, tuberculosis, diphtheria and cholera. These diseases can be treated with ANTIBIOTICS although there is a problem of resistance of some bacteria to certain antibiotic drugs.

Taken from Dictionary of Science

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The bacteria ([bækˈtɪərɪə]; singular: bacterium) are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×10³⁰) bacteria on Earth, forming much of the world's biomass. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. However, most bacteria have not been characterized, and only about half of the phyla of bacteria have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

There are approximately ten times as many bacterial cells in the human flora of bacteria as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and a few are beneficial. However, a few species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people a year, mostly in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and in agriculture, so antibiotic resistance is becoming common. In industry, bacteria are important in sewage treatment, the production of cheese and yoghurt through fermentation, as well as in biotechnology, and the manufacture of antibiotics and other chemicals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved independently from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.

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Taxis

(plural taxes) the movement of a cell or organism in response to a stimulus in environment. This stimulus may be temperature (thermotaxis), light (phototaxis), gravity (geotaxis) or chemical (CHEMOTAXIS).

Taken from Dictionary of Science

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A taxis (plural taxes, pronounced /ˈtæksiːz/) is an innate behavioral response by an organism to a directional stimulus or gradient of stimulus intensity. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that the organism has motility and demonstrates guided movement towards or away from the stimulus source. It is sometimes distinguished from a kinesis, a non-directional change in activity in response to a stimulus that results in the migration toward or away from a stimulus.

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Macrophage

a special type of cell that forms part of the IMMUNE SYSTEM of vertebrates. Macrophages are derived from MONOCYTES in the blood system and can move to infected or inflamed areas of the body using PSEUDOPODIA. In these areas, they surround, engulf and degrade broken cells and other debris, including microbes, by means of the process called PHAGOCYTOSIS.

Taken from Dictionary of Science