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Adapted by Mary Shomon

Acquired immunodeficiency syndrome (AIDS):

A life-threatening disease caused by a virus and characterized by breakdown of the body's immune defenses.

Active immunity:

Immunity produced by the body in response to stimulation by a disease-causing organism or a vaccine.

Agammaglobulinemia:

An almost total lack of immunoglobulins, or antibodies.

Allergen:

Any substance that causes an allergy.

Allergy:

An inappropriate and harmful response of the immune system to normally harmless substances.

Anaphylactic shock:

A life-threatening allergic reaction characterized by a swelling of body tissues including the throat, difficulty in breathing, and a sudden fall in blood pressure.

Anergy:

A state of unresponsiveness, induced when the T cell's antigen receptor is stimulated, that effectively freezes T cell responses pending a "second signal" from the antigen-presenting cell (co-stimulation).

Antibody:

A soluble protein molecule produced and secreted by B cells in response to an antigen, which is capable of binding to that specific antigen.

Antibody-dependent cell-mediated cytotoxicity (ADCC):

An immune response in which antibody, by coating target cells, makes them vulnerable to attack by immune cells.

Antigen:

Any substance that, when introduced into the body, is recognized by the immune system.

Antigen-presenting cells:

B cells, cells of the monocyte lineage (including macrophages as well as dendritic cells), and various other body cells that "present" antigen in a form that T cells can recognize.

Antinuclear antibody (ANA):

An autoantibody directed against a substance in the cell's nucleus.

Antiserum:

Serum that contains antibodies.

Antitoxins:

Antibodies that interlock with and inactivate toxins produced by certain bacteria.

Appendix:

Lymphoid organ in the intestine.

Attenuated:

Weakened; no longer infectious.

Autoantibody:

An antibody that reacts against a person's own tissue.

Autoimmune disease:

A disease that results when the immune system mistakenly attacks the body's own tissues. Rheumatoid arthritis and systemic lupus erythematosus are autoimmune diseases.

Bacterium:

A microscopic organism composed of a single cell. Many but no all bacteria cause disease.

Basophil:

A white blood cell that contributes to inflammatory reactions. Along with mast cells, basophils are responsible for the symptoms of allergy.

B cells:

Small white blood cells crucial to the immune defenses. Also known as B lymphocytes, they are derived from bone marrow and develop into plasma cells that are the source of antibodies.

Biological response modifiers:

Substances, either natural or synthesized, that boost, direct, or restore normal immune defenses. BRMs include interferons, interleukins, thymus hormones, and monoclonal antibodies.

Biotechnology:

The use of living organisms or their products to make or modify a substance. Biotechnology includes recombinant DNA techniques (genetic engineering) and hybridoma technology.

Bone marrow:

Soft tissue located in the cavities of the bones. The bone marrow is the source of all blood cells.

Cellular immunity:

Immune protection provided by the direct action of immune cells (as distinct from soluble molecules such as antibodies).

Chromosomes:

Physical structures in the cell's nucleus that house the genes. Each human cell has 23 pairs of chromosomes.

Clone:

(n.)A group of genetically identical cells or organisms descended from a single common ancestor; (v.) to reproduce multiple identical copies.

Complement:

A complex series of blood proteins whose action "complements" the work of antibodies. Complement destroys bacteria, produces inflammation, and regulates immune reactions.

Complement cascade:

A precise sequence of events usually triggered by an antigen-antibody complex, in which each component of the complement system is activated in turn.

Constant region:

That part of an antibody's structure that is characteristic for each antibody class.

Co-Stimulation:

The delivery of a second signal from an antigen-presenting cell to a T cell. The second signal rescues the activated T cell from anergy, allowing it to produce the lymphokines necessary for the growth of additional T cells.

Cytokines:

Powerful chemical substances secreted by cells. Cytokines include lymphokines produced by lymphocytes and monokines produced by monocytes and macrophages.

Cytotoxic T cells:

A subset of T lymphocytes that can kill body cells infected by viruses or transformed by cancer.

Dendritic cells:

White blood cells found in the spleen and other lymphoid organs. Dendritic cells typically use threadlike tentacles to enmesh antigen, which they present to T cells.

DNA (deoxyribonucleic acid):

Nucleic acid that is found in the cell nucleus and that is the carrier of genetic information.

Enzyme:

A protein, produced by living cells, that promotes the chemical processes of life without itself being altered.

Eosinophil:

A white blood cell that contains granules filled with chemicals damaging to parasites, and enzymes that damp down inflammatory reactions.

Epitope:

A unique shape or marker carried on an antigen's surface, which triggers a corresponding antibody response.

Fungus:

Member of a class of relatively primitive vegetable organism. Fungi include mushrooms, yeasts, rusts, molds, and smuts.

Gene:

A unit of genetic material (DNA) that carries the directions a cell uses to perform a specific function, such as making a given protein.

Graft-versus-host disease (GVHD):

A life-threatening reaction in which transplanted immunocompetent cells attack the tissues of the recipient.

Granulocytes:

White blood cells filled with granules containing potent chemicals that allow the cells to digest microorganisms, or to produce inflammatory reactions. Neutrophils, eosinophils, and basophils are examples of granulocytes.

Helper T cells:

A subset of T cells that typically carry the T4 marker and are essential for turning on antibody production, activating cytotoxic T cells, and initiating many other immune responses.

Hematopoiesis:

The formation and development of blood cells, usually takes place in the bone marrow.

Histocompatibility testing:

A method of matching the self antigens (HLA) on the tissues of a transplant donor with those of the recipient. The closer the match, the better the chance that the transplant will take.

HIV (human immunodeficiency virus):

The virus that causes AIDS.

Human leukocyte antigens (HLA):

Protein in markers of self used in histocompatibility testing. Some HLA types also correlate with certain autoimmune diseases.

Humoral immunity:

Immune protection provided by soluble factors such as antibodies, which circulate in the body's fluids or "humors," primarily serum and lymph.

Hybridoma:

A hybrid cell created by fusing a B lymphocyte with a long-lived neoplastic plasma cell, or a T lymphocyte with a lymphoma cell. A B-cell hybridoma secretes a single specific antibody.

Hypogammaglobulinemia:

Abno rmally low levels of immunoglobulins.

Idiotypes:

The unique and characteristic parts of an antibody's variable region, which can themselves serve as antigens.

Immune complex:

A cluster of interlocking antigens and antibodies.

Immune response:

The reactions of the immune system to foreign substances.

Immunoassay:

A test using antibodies to identify and quantify substances. Often the antibody is linked to a marker such as a fluorescent molecule, a radioactive molecule, or an enzyme.

Immunocompetent:

Capable of developing an immune response.

Immunoglobulins:

A family of large protein molecules, also known as antibodies.

Immunosuppression:

Reduction of the immune responses, for instance by giving drugs to prevent transplant rejection.

Immunotoxin:

A monoclonal antibody linked to a natural toxin, a toxic drug, or a radioactive substance.

Inflammatory response:

Redness, warmth, swelling, pain, and loss of function produced in response to infection, as the result of increased flood flow and an influx of immune cells and secretions.

Interleukins:

A major group of lymphokines and monokines.

Kupffer cells:

Specialized macrophages in the liver.

LAK cells:

Lymphocytes transformed in the laboratory into lymphokine-activated killer cells, which attack tumor cells.

Langerhans cells:

Dendritic cells in the skin that pick up antigen and transport it to lymph nodes.

Leukocytes:

All white blood cells.

Lymph:

A transparent, slightly yellow fluid that carries lymphocytes, bathes the body tissues, and drains into the lymphatic vessels.

Lymphatic vessels:

A bodywide network of channels, similar to the blood vessels, which transport lymph to the immune organs and into the bloodstream.

Lymph nodes:

Small bean-shaped organs of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells.

Lymphocytes:

Small white blood cells produced in the lymphoid organs and paramount in the immune defenses.

Lymphoid organs:

The organs of the immune system, where lymphocytes develop and congregate. They include the bone marrow, thymus, lymph nodes, spleen, and various other clusters of lymphoid tissue. The blood vessels and lymphatic vessels can also be considered lymphoid organs.

Lymphokines:

Powerful chemical substances secreted by lymphocytes. These soluble molecules help direct and regulate the immune responses.

Macrophage:

A large and versatile immune cell that acts as a microbe-devouring phagocyte, an antigen-presenting cell, and an important source of immune secretions.

Major histocompatibility complex (MHC):

A group of genes that controls several aspects of the immune response. MHC genes code for self markers on all body cells.

Mast cell:

A granule-containing cell found in tissue. The contents of mast cells, along with those of basophils, are responsible for the symptoms of allergy.

Microbes:

Minute living organisms, including bacteria, viruses, fungi and protozoa.

Microorganisms:

Microscopic plants or animals.

Molecule:

The smallest amount of a specific chemical substance that can exist alone. (The break a molecule down into its constituent atoms is to change its character. A molecule of water, for instance, reverts to oxygen and hydrogen.)

Monoclonal antibodies:

Antibodies produced by a single cell or its identical progeny, specific for a given antigen. As a tool for binding to specific protein molecules, monoclonal antibodies are invaluable in research, medicine, and industry.

Monocyte:

A large phagocytic white blood cell which, when it enters tissue, develops into a macrophage.

Monokines:

Powerful chemical substances secreted by monocytes and macrophages. These soluble molecules help direct and regulate the immune responses.

Natural killer (NK) cells:

Large granule-filled lymphocytes that take on tumor cells and infected body cells. They are known as "natural" killers because they attack without first having to recognize specific antigens.

Neutrophil:

A white blood cell that is an abundant and important phagocyte.

Nucleic acids:

Large, naturally occurring molecules composed of chemical building blocks known as nucleotides. There are two kinds of nucleic acids, DNA and RNA.

OKT3:

A monoclonal antibody that targets mature T cells.

Opportunistic infection:

An infection in an immunosuppressed person caused by an organism that does not usually trouble people with healthy immune systems.

Opsonize:

To coat an organism with antibodies or a complement protein so as to make it palatable to phagocytes.

Organism:

An individual living thing.

Parasite:

A plant or animal that lives, grows and feeds on or within another living organism.

Passive immunity:

Immunity resulting from the transfer of antibodies or antiserum produced by another individual.

Peyer's patches:

A collection of lymphoid tissues in the intestinal tract.

Phagocytes:

Large white blood cells that contribute to the immune defenses by ingesting microbes or other cells and foreign particles.

Plasma cells:

Large antibody-producing cells that develop from B cells.

Platelets:

Granule-containing cellular fragments critical for blood clotting and sealing off wounds. Platelets also contribute to the immune response.

Polymorphs:

Short for polymorphonuclear leukocytes or granulocytes.

Proteins:

Organic compounds made up of amino acids. Proteins are one of the major constituents of plant and animal cells.

Protozoa:

A group of one-celled animals, a few of which cause human disease (including malaria and sleeping sickness).

Rheumatoid factor:

An autoantibody found in the serum of most persons with rheumatoid arthritis.

RNA (ribonucleic acid):

A nucleic acid that is found in the cytoplasm and also in the nucleus of some cells. One function of RNA is to direct the synthesis of proteins.

Scavenger cells:

Any of a diverse group of cells that have the capacity to engulf and destroy foreign material, dead tissues, or other cells.

SCID mouse:

A laboratory animal that, lacking an enzyme necessary to fashion an immune system of its own, can be turned into a model of the human immune system when injected with human cells or tissues.

Serum:

The clear liquid that separates from the blood when it is allowed to clot. This fluid retains any antibodies that were present in the whole blood.

Severe combined immunodeficiency disease (SCID):

A life-threatening condition in which infants are born lacking all major immune defenses.

Spleen:

A lymphoid organ in the abdominal cavity that is an important center for immune system activities.

Stem cells:

Cells from which all blood cells derive. The bone marrow is rich in stem cells.

Subunit vaccine:

A vaccine that uses merely one component of an infectious agent, rather than the whole, to stimulate an immune response.

Superantigens:

A class of antigens, including certain bacterial toxins, that unleash a massive and damaging immune response.

Suppressor T cells:

A subset of T cells that turn off antibody production and other immune responses.

T cells:

Small white blood cells that orchestrate and/or directly participate in the immune defenses. Also known as T lymphocytes, they are processed in the thymus and secrete lymphokines.

Thymus:

A primary lymphoid organ, high in the chest, where T lymphocytes proliferate and mature.

TIL:

Tumor-infiltrating lymphocytes. These immune cells are extracted from the tumor tissue, treated in laboratory, and reinjected into the cancer patient.

Tissue typing:

See histocompatibility testing.

Tolerance:

A state of nonresponsiveness to a particular antigen or group of antigens.

Tonsils and adenoids:

Prominent oval masses of lymphoid tissues on either side of the throat.

Toxins:

Agents produced by plants and bacteria, normally very damaging to mammalian cells, that can be delivered directly to target cells by linking them to monoclonal antibodies or lymphokines.

Vaccine:

A substance that contains antigenic components from an infectious organism. By stimulating an immune response (but not disease), it protects against subsequent infection by that organism.

Variable region:

That part of an antibody's structure that differs from one antibody to another.

Virus:

Submicroscopic microbe that causes infectious disease. Viruses can reproduce only in living cells.