Lymphatic and Immune System Terms

Allergen: An antigen that induces an allergic or hypersensitivity reaction.

Allergy: Hypersensitivity caused by exposure to an allergen that, on subsequent exposure, often results in harmful immunologic consequences, such as severe inflammation or tissue damage.

Antibody (Ab): A protein molecule synthesized by plasma cells that are derived from B lymphocytes in response to the introduction of an antigen. Antibodies are divided into five kinds (IgA, IgD, IgE, IgG, IgM). Also called immunoglobulin.

Antigen (Ag): A substance capable of eliciting an immune response, that reacts with immune cells or antibodies; antigen is a contraction of antibody generator.

Antigenic determinant: The particular site on an antigen that binds to a particular antibody and determines immunologic specificity; many antigens have several different antigenic determinants, and each binds to a different antibody. Also called an epitope.

Antigen presenting cell (APC): Special class of migratory cell that processes and presents antigens to lymphocytes during an immune response; includes macrophages, dendritic cells, and B cells.

Appendix: A wormlike tube attached to the cecum that contains masses of lymphoid tissue.

Autoimmune disease: Any disorder in which normal tissue is destroyed by the production of antibodies or T cells that attack the body's own tissue.

Basophil: Granular white blood cell that releases histamine and other mediators of inflammation; contains the anticoagulant heparin; its granules take up basic stain and turn blueish.

B cell: A type of lymphocyte that becomes immunocompetent in bone marrow; when properly stimulated by a specific antigen, it can develop into a clone of antibody producing plasma cells or memory B cells.

Chemotaxis: Movement of cells toward a chemical substance, such as white blood cells moving toward inflammatory chemicals.

Cisterna chyli: Large lymph sac on the bodies of the L1 and L2 vertebrae that receives lymph drainage from the digestive organs via the intestinal trunk, and from the two lumbar lymphatic trunks.

Complement: A group of blood proteins which, when activated, forms a membrane attack complex that leads to cytolysis of the target cell, and enhances the inflammatory and immune responses.

Cytokine: Hormonelike protein involved in cell-mediated immune responses, such as interferon, interleukin, and lymphokine.

Cytotoxic T cell: A type of T lymphocyte that kills foreign cells, cancer cells, and virus infected body cells.

Dendritic cell: A type of antigen-presenting cell with long branchlike projections, found in the skin, mucosal linings, and lymph nodes.

Diapedesis: Passage of white blood cells through intact blood vessel walls into tissue.

Eosinophil: Granular white blood cell that kills parasitic worms and destroys antigen-antibody complexes; its granules take up an acid stain called eosin and stain red.

Hapten: A small molecule that is not antigenic by itself, but when bound to a carrier protein, it can stimulate antibody production and it can activate T cells. Also called an incomplete antigen.

Helper T cell: A type of T lymphocyte that helps cellular immunity by direct contact with other lymphocytes, and by releasing cytokines.

Histamine: Substance that causes vasodilation, increased vascular permeability, and constriction of bronchioles; found in basophils, mast cells, and platelets, that release it when cells are injured.

Immunocompetent: The ability to mount a normal immune response.

Inflammation: Protective response to tissue injury characterized by redness, swelling, pain, and heat.

Interferon (IFN): Cytokines released by virus infected cells that helps protect body cells from viral replication.

Interstitial fluid (IF): Fluid between the cells. Also called intercellular or tissue fluid.

Lymph: Fluid in lymphatic vessels that is returned to the blood.

Lymph node: Small bean-shaped lymphatic organ that filters lymph; large clusters occur in the inguinal, axillary, and cervical regions; it is surrounded by a fibrous capsule with extensions called trabeculae that divide the cortex into compartments; macrophages and dendritic cells are found throughout the node; the outer cortex contains follicles with germinal centers where B cells divide; the deeper cortex contains T cells; the medulla contains activated B cells and plasma cells.

Lymphatic capillary: Closed-ended vessel with flaplike minivalves that begins in spaces between cells and returns lymph to the blood.

Lymphatic vessels: The vessels that convey the lymph, which anastomose freely with each other.

Lymphocyte: Agranular white blood cell that helps carry out immune responses.

Macrophage: Actively phagocytic cell that is widely distributed in the body; participates in presenting antigens to lymphocytes; derived from monocytes.

Major histocompatibility complex (MHC): Surface glycoproteins on nucleated cells that are involved in immune responsiveness to antigens, and compatibility of transplanted tissues.

Mast cell: Immune cell found in connective tissue that releases histamine during inflammation, and is involved in hypersensitivity (allergic) reactions.

Memory cell: Clone of a B or T cell that provides enhanced immune reactions when reexposed to an antigen.

Monocyte: Agranular white blood cell; phagocyte that develops into a macrophage in tissues.

Mucosa-associated lymphatic tissue (MALT): Lymphatic nodules scattered throughout the lamina propria (connective tissue) of mucous membranes lining the gastrointestinal tract, respiratory airways, urinary tract, and reproductive tract.

Natural killer cell: Large granular lymphocyte capable of killing a cell without prior sensitization to it; it can recognize and kill cells that present unusual plasma membrane proteins; it destroys cells that have been opsonized with antibodies. Also called NK cell.

Neutrophil: Granular white blood cell; active phagocyte that increases rapidly during acute infection; the most abundant type of white blood cell; its granules take up a mixture of acidic and basic stains and turn pale lilac.

Opsonization: The binding of antibodies and/or complement to cells, which enhances phagocytosis.

Peyer patches: Collections of lymphoid follicles in the ileum of the small intestine.

Plasma: The noncellular fluid portion of the circulating blood.

Plasma cell: Cell that develops from a B lymphocyte (B cell) and produces antibodies.

Red bone marrow: A primary lymphoid organ that produces all of the blood cells, and the site where B cells mature and develop immunocompetence; located in spaces between trabeculae of spongy bone tissue in the flat bones and epiphyses of long bones.

Reticular connective tissue: Tissue with many cell types on a network of reticular fibers in a typical loose ground substance. Functions as the internal skeleton (stroma) of organs, protection against disease, and removal of worn-out cells. Located in lymphoid organs, such as lymph nodes, spleen, and bone marrow.

Right lymphatic duct: Large lymph vessel that drains lymph from the right arm and the right side of the head and thorax, and empties into the right brachiocephalic vein.

Septicemia: Disease caused by the spread of pathogens and their toxins through circulating blood. Also called sepsis, or blood poisoning.

Spleen: Largest lymphoid organ; provides for blood cleansing, immune surveillance and response, and lymphocyte proliferation.

T cell: A type of lymphocyte that becomes immunocompetent in the thymus; when properly stimulated by a specific antigen, it can develop into a clone of helper T cells, cytotoxic T cells, or other types of T cells, which function in cell-mediated immunity.

Thoracic duct: The largest lymph vessel in the body that drains lymph from the left side of the body, the abdomen, and both lower limbs; it begins at the cisterna chyli and empties into the left brachiocephalic vein. Also called the left lymphatic duct.

Thymus: A primary lymphoid organ and endocrine gland that produces the hormones thymosin, thymopoietin, and thymulin (serum thymic factor); it stimulates the differentiation, maturation, and immunocompetence of T cells, and enhances the actions of T-cells and natural killer cells.

Tonsils: Five collections of lymphoid follicles that collectively form a ring in the oropharynx; includes the pharyngeal tonsil (adenoid), palatine tonsils, and lingual tonsils.