Antibody-dependent Killer Cell: This cell is similar to a natural killer cell, except action requires triggering by antibody coating of target cells, mediated by Fc receptors. The cell may be derived from lymphocytes, polymorphs, eosinophils, or monocytes/macrophages.

Basophils: A granulocyte named for its ability to be stain by basic dyes. It contains vasoactive amines such as histamine and serotonin, which are released when the cell degranulates.

CD4 receptor: A glycoprotein on the surface of one kind of mature T cell as well as immature thymocytes, and in smaller quantities on macrophages (monocytes). CD4 is a signalling protein and is involved in the regulation of communication between and activation of T cells and B cells. HIV uses its gp120 surface to bind to the side of CD4 receptors on T cells as the first step in invading them.

Chemokine receptor: A protein on a cell surface that receives (binds to) chemokines. HIV binds to some chemokine receptors as its second step in invading a T4 cell.

Chemokine: A family of structurally related immune system hormones with potent leukocyte activation and/or chemotaxis. They are called 'pro-inflammator' molecules because they attract white blood cells to themselves, which can result in localized swelling and inflammation.

Chemotaxis: Moving from one place to another because of a chemical signal. An example of chemotaxis is a white blood cell moving to the site of an infection in response to immune system hormones produced near the infection.

Cross-linking: Linking across from one molecule to another, usually identical, molecule. The action binds the two cross-linked molecules together. In the human body, cross-linking molecules which were not intended to be linked to each other can cause unexpected behavior.

Cytokine: Any of many different immune system hormones, such as Interleukin-2.

Enzyme: A protein which changes or affects other proteins. For example, an human enzyme named furin cuts specific proteins into two pieces.

Eosinophils: A granulocyte which is readily stained by eosin. Its granules are coarse, round, and uniform in size; the nucleus usually has two lobes connected by a slender thread of chromatin. These cells are thought to play a specialized role in immunity to helminth infections. Previously called eosinocytes.

Granulocyte: A white blood cell which contains granules, or small pockets of chemicals which, under a microscope, look somewhat similar in shape to grains of sand. Basophils, eosinophils, mast cells, myelocytes, and neutrophils are all kinds of granulocytes.

Hormone: A chemical which circulates through the body and affects the activity of cells.

Interferon: a group of proteins produced by infected host cells that reduce the likelihood of viral infection for nearby cells. As with many immune system hormones, low concentrations produce different results than high concentrations. There are three identified Interferons, named alpha, beta, and gamma. Interferon-gamma, for example, is secreted in significant quantity during a viral "stomach flu" infection. Its side effects are unpleasant.

Interleukin-10: An immune system hormone which increases the viability of B cells. It inhibits the production of immune system hormones by activated T cells. Usually abbreviated IL-10.

Interleukin-2: An immune system hormone which stimuates T cells. It is produced by T cells in response to antigenic or mitogenic stimulation and the signal carried by Interleukin-1. It stimulates the proliferation of T cells bearing specific receptors for itself. It is given as a drug to late-stage AIDS patients to boost T cell activity. Its side effects include flu-like symptoms. It is usually abbreviated IL-2. It has previously been called Aldesleukin, lymphocyte transforming factor (LTF), T cell growth factor (TCGF) and thymocyte stimulating factor (TSF).

Interleukin-4: An immune system hormone which is produced by antigen- or mitogen-activated T lymphocytes to enhance the ability of B lymphocytes to present antigen and to prime B lymphocytes to proliferate in response to anti-Ig and lipopolysaccharide. It regulates the production of IgG and IgE by B lymphocytes and also acts as a growth factor for T lymphocytes and as a maturation factor for mast cells. It has previously been called B cell stimulating factor 1 (BSF1) and B cell growth factor (BCGF). Usually abbreviated IL-4.

Interleukin-8: A basic, heparin-binding, proinflammatory immune system hormone. It is produced by monocytes, vascular endothelium, and other cell types. It acts on neutrophils as a chemoattractant, activator, and modulator of endothelial adhesion and transmigration. Usually abbreviated IL-8.

Leukocyte: A white blood cell. A part of the immune system.

Macrophage: A phagocyte with one nucleus which is derived from a monocyte and which is found in tissue. It may be fixed or freely moving. It protects against infection and noxious substances. Its previous names are many and varied, usually depending on the type of tissue that it was isolated in.

Mast cell: A granulated cell found abundantly in the skin and the gastrointestinal tract. Mast cells contain large amounts of histamine and heparin, but unlike basophils, they normally remain in the tissues and do not circulate in the blood. Mast cell degranulation (dumping out the contents of internal granules) can cause the symptoms of a severe allergic reaction (anaphylaxis).

Monocytes: A large white blood cell with finely granulated chromatin dispersed throughout the nucleus. It is formed in the bone marrow, enters the blood, and migrates into the connective tissue where it differentiates into a macrophage.

Natural Killer Cell: A type of cytotoxic leukocyte which attacks and kills virus-infected or neoplastic (cancerous) cells independent of antibody coating or MHC restriction; it may be derived from lymphocytes or monocytes.

Neutrophils: White blood cells with a nucleus with three to five lobes connected by slender threads of chromatin, and cytoplasm containing fine inconspicuous granules and stainable by neutral (non-acid/non-basic) dyes. Its name is derived from this last quality. It is both a granulocyte and a phagocyte.

Phagocyte: A cell that ingests microorganisms, foreign particles, other cells, or dead tissue.

T cell: A kind of white blood cell. T cells mature in the thymus and derive their name from that organ. They are subdivided into T4 cells and T8 cells.

T4 cell: A T cell with CD4 receptors on their surface. Also called Helper T cells or CD4+ T cells. They regulate much of the immune system.

T4 cell count or CD4 count: Most HIV+ patients check the number of T4 cells in their bloodstream. A normal count might be between 800 and 1200 T4 cells per cubic millimeter (usually written mm3 or femtoliter). Single tests are less important than the overall trend. A sustained count below 400 suggests that the immune system may be unable to fight off infections without help. A count below 200 indicates the need for a review for a formal AIDS diagnosis.

T8 cell: A T cell with CD8 receptors on their surface. They kill cells infected by viruses. They are antibody-independent "killer cells" which, when stimulated by antigen, MHC class I antigen, and interleukin 2, secrete cytolytic pore-forming proteins capable of lysing target cells. Also called Killer T cells or Cytotoxic T cells.

Vascular endothelial cell: Flat, interleaving cells lining the inside of a vein. In response to Interleukin-2, they pull apart from each other to form a small space big enough for a white blood cell to squeeze through. Some toxins mimic IL-2 and cause large holes to form, with the result that fluid leaks out of the bloodstream and the patient goes into shock.


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