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Here is a glossary of commonly used phrases in immunology.

The Immune System: Glossary

A | B | C | D-G | H-L | M-N | O-S | T-V

adrenal gland – a gland located on each kidney that secretes hormones regulating metabolism, sexual function, water balance, and stress.

allergen – any substance that causes an allergy.

allergy – a harmful response of the immune system to normally harmless substances.

antibodies – molecules (also called immunoglobulins) produced by a B cell in response to an antigen. When an antibody attaches to an antigen, it helps the body destroy or inactivate the antigen.

antigen – a substance or molecule that is recognized by the immune system. The molecule can be from foreign material such as bacteria or viruses.

antiserum – a serum rich in antibodies against a particular microbe.

appendix – lymphoid organ in the intestine.

autoantibodies – antibodies that react against a person's own tissue.

autoimmune disease – disease that results when the immune system mistakenly attacks the body's own tissues. Examples include multiple sclerosis, type I diabetes, rheumatoid arthritis, and systemic lupus erythematosus.


B cells – small white blood cells crucia to the immune defenses. Also know as B lymphocytes, they come from bone marrow and develop into blood cells called plasma cells, which are the source of antibodies.

bacteria – microscopic organisms composed of a single cell. Some cause disease.

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

biological response modifiers – substances, either natural or synthesized, that boost, direct, or restore normal immune defenses. They include interferons, interleukins, thymus hormones, and monoclonal antibodies.

blood vessels – arteries, veins, and capillaries that carry blood to and from the heart and body tissues.

bone marrow – soft tissue located in the cavities of the bones. Bone marrow is the source of all blood cells.


chemokines – certain proteins that stimulate both specific and general immune cells and help coordinate immune responses and inflammation.

clone – a group of genetically identical cells or organisms descended from a single common ancestor; or, to reproduce 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 antigen-antibody complexes, in which each component of the complement system is activated in turn.

cytokines – powerful chemical substances secreted by cells that enable the body's cells to communicate with one another. Cytokines include lymphokines produced by lymphocytes and monokines produced by monocytes and macrophages.

cytotoxic T lymphocytes (CTLs) – a subset of T cells that carry the CD8 marker and can destroy body cells infected by viruses or transformed by cancer.


DNA (deoxyribonucleic acid) – a long molecule found in the cell nucleus; it carries the cell's genetic information.

enzyme – a protein produced by living cells that promotes the chemical processes of life without itself being altered.

eosinophils – white blood cells that contain granules filled with chemicals damaging to parasites, and enzymes that affect inflammatory reactions.

epithelial cells – cells making up the epithelium, the covering for internal and external body surfaces.

fungi – members of a class of relatively primitive vegetable organisms. They include mushrooms, yeasts, rusts, molds, and smuts.

genes – units of genetic material (DNA) inherited from a parent. Genes carry the directions a cell uses to perform a specific function.

graft rejection – an immune response against transplanted tissue.

graft-versus host disease (GVHD) – a life-threatening reaction in which transplanted cells attack the tissues of the recipient.

granules – membrane-bound organelles within cells where proteins are stored before secretion.

granulocytes – phagocytic white blood cells filled with granules organisms. Neutrophils, eosinophils, basophils, and mast cells are examples of granulocytes.

growth factors – chemicals secreted by cells that stimulate proliferation of or changes in the physical properties of other cells.


helper T cells (Th cells) – a subset of T cells that carry the CD4 surface marker and are essential for turning on antibody production, activating cytotoxic T cells, and initiating many other immune functions.

HIV (human immunodeficiency virus) – the virus that causes AIDS.

immune response – reaction of the immune system to foreign substances.

immunoglobulins – a family of large protein molecules, also known as antibodies, produced by B cells.

immunosuppressive – capable of reducing immune responses.

inflammatory response – redness, warmth, and swelling produced in response to infection, as the result of increased blood flow and an influx of immune cells and secretions.

interferons – proteins produced by cells that stimulate anti-virus immune responses or alter the physical properties of immune cells.

interleukins – a major group of lymphokines and monokines.

leukocytes – all white blood cells.

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

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.

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

lymphocytes – small white blood cells produced in the lymphoid organs and paramount in the immune defenses. B cells and T cells are lymphocytes.

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. Blood vessels and lymphatic vessels are also lymphoid organs.

lymphokines – powerful chemical substances secreted by lymphocytes. These molecules help direct and regulate the immune responses.


macrophage – a large and versatile immune cell that devours invading pathogens and other intruders. Macrophages stimulate other immune cells by presenting them with small pieces of the invaders.

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 granulocyte found in tissue. The contents of mast cells, along with those of basophils, are responsible for the symptoms of allergy.

memory cells – a subset of T cells and B cells that have been exposed to antigens and can then respond more readily when the immune system encounters those same antigens again.

microbes – microscopic living organisms, including bacteria, viruses, fungi, and protozoa.

microorganisms – microscopic organisms, including bacteria, virus, fungi, plants, and parasites.

molecule – the smallest amount of a specific chemical substance. Large molecules such as proteins, fats, carbohydrates, and nucleic acids are the building blocks of a cell, and a gene determines how each molecule is produced.

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

monocytes – large phagocytic white blood cells which, when entering tissue, develop into macrophages.

monokines – powerful chemical substances secreted by monocytes and macrophages. These molecules help direct and regulate the immune responses.

natural killer (NK) cells – large granule-containing lymphocytes that recognize and kill cells lacking self antigens. Their target recognition molecules are different from T cells.

neutrophil – white blood cell that is an abundant and important phagocyte.


organisms – individual living things.

parasites – plants or animals that live, grow, and feed on or within another living organism.

passive immunity – immunity resulting from the transfer of antibodies or antiserum produced by another individual.

pathogen – a disease-causing organism.

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

phagocytosis – process by which one cell engulfs another cell or large particle.

plasma cells – large antibody-producing cells that develop from B cells.

platelet – cellular fragment critical for blood clotting and sealing off wounds.

serum – the clear liquid that separates from the blood when it is allowed to clot. This fluid contains the antibodies that were present in the whole blood.

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

stem cells – immature cells from which all cells derive. The bone marrow is rich in stem cells, which become specialized blood cells.


T cells – small white blood cells (also known as T lymphocytes) that recognize antigen fragments bound to cell surfaces by specialized antibody-like receptors. "T" stands for thymus, where T cells acquire their receptors.

T lymphocytes – see T cells.

thymus – a primary lymphoid organ, high in the chest, where T lymphocytes proliferate and mature.

tissue typing – see histocompatibility testing.

tissues – groups of similar cells joined to perform the same function.

tolerance – a state of immune 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 in plants and bacteria, normally very damaging to cells.

vaccines – preparations that stimulate an immune response that can prevent an infection or create resistance to an infection. They do not cause disease.

viruses – microorganisms composed of a piece of genetic material — RNA or DNA — surrounded by a protein coat. Viruses can reproduce only in living cells.


Source: Understanding the Immune System: How It Works. U.S. Department of Health and Human Services & The National Institutes of Health (National Institute of Allergy and Infectious Diseases, National Cancer Institute). NIH Publication No. 03-5423. September, 2003, pages 47-57.