adaptive immune response: also known as "acquired immunity," an arm of the immune system involving immune cells (B and T cells) which provide a specific and sustained immune defense against foreign invaders.

antibody: a protein produced by certain immune cells (called B cells) that help fight and destroy viruses, bacteria, and other foreign substances that invade the body.

antigen: a substance or molecule that is recognized by the immune system and stimulates an immune response. The molecule can be from a foreign material such as a bacterium or virus, or in people with autoimmune disease, the body's own cells may be seen as antigens.

antigen-presenting cell: a cell that processes an antigen with an MHC molecule on the cell surface.

antiphospholipid syndrome: a condition that occurs in about 20-30% of people with systemic lupus erythematosus who develop abnormal antibodies that can cause blood clots to form and which can result in heart attacks, strokes and miscarriages.

autoantibody: abnormal antibodies that are made against the body's own organs and tissues rather than foreign parts of bacteria or viruses.

autoimmune disease: condition in which the immune system mistakenly attacks the body's own organs and tissues.

B cell: a type of lymphocyte, which is an immune system cell. Among its many roles, the B cell produces antibodies that bind antigens.

biologic response modifiers: new class of treatment agents that selectively target a specific component of the immune system. For instance, biologic agents that target the cytokines tumor necrosis factor alpha and interleukin-1 have been shown in clinical trials to reduce joint inflammation and damage.

cells: the building blocks that make up tissues, organs, and bloodstream of the body. Immune system cells normally move throughout the bloodstream and reside temporarily in the lymph nodes, spleen, and thymus.

chemokine: a substance manufactured by cells and tissues, that stimulates movement and activation of immune system cells to the area where the chemokine is produced.

clinical trial: a prospective, scientific evaluation in human volunteers of a treatment regimen, device, or procedure used for the prevention, diagnosis, or treatment of a disease.

complement system: this series of molecules works together to perform many immune system functions. For example, the complement system helps to dissolve and remove immune complexes and to kill foreign cells.

co-stimulatory molecules: pairs of molecules on the surfaces of two cells that work together with the MHC and T-cell receptors of those cells. The co-stimulatory molecules help to stimulate or decrease the immune response produced by the two cells.

cytokines: chemical substances that act as messengers between cells and can have varied effects on many cells of the body. For example, some cytokines can cause growth and activation of immune system cells. Two cytokines that play a key role in rheumatoid arthritis are tumor necrosis factor alpha and interleukin-1.

dendritic cells: a specialized type of cell that can present antigens to immune cells and which display an extraordinary capacity to stimulate T cells and initiate an immune response.

gene: a unit of genetic material that is inherited from a parent. A gene carries the directions that a cell uses to perform a specific function.

immune complex: a cluster of interlocking antigens and antibodies forming a large network of molecules.

immune response: the reaction of the immune system against foreign substances. When the reaction occurs against the body's own cells or tissues, it is called an autoimmune reaction.

immune system: a complex system that normally protects the body from infections. The immune system consists of a group of cells, the chemicals that control those cells, and the chemicals that those cells release.

immunosuppressive drugs: drugs that suppress the immune response and can be used to treat autoimmune disease. Because normal immunity is also suppressed with these drugs, they leave the body at risk for infection.

infection: invasion of the body tissues by bacteria or other tiny organisms that cause illness.

inflammation: a collection of immune system cells and molecules that invade tissues and organs as part of an immune system response to injury or disease, typically marked by four signs: swelling, redness, heat, and pain.

innate immune response: the body's first line of defense against foreign invaders such as bacteria and viruses, consisting of dendritic cells, macrophages, natural killer cells, complement and other immune components that provide an immediate, non-specific response.

lymph nodes: immune system organs spread throughout the body that help defend against disease.

lymphocyte: a type of white blood cell of the immune system. T cells and B cells are lymphocytes that look similar under the microscope but have different functions.

macrophage: a type of white blood cell that functions as a patrol cell and engulfs and kills foreign infectious invaders.

MHC (major histocompatibility complex) molecules: molecules that are found on cell surfaces and display antigen; the antigen-MHC molecules may then interact with a T-cell receptor.

molecule: a small physical unit made up of chemical substances such as proteins, sugars or fats. Molecules are the building blocks of a cell.

monoclonal antibody: synthetic antibody molecules that are mass-produced in the laboratory and have the ability to target and attach to specific molecules in the body. They are used in laboratory research, medical tests, and as therapeutic agents.

neutrophil: a type of immune system cell that combats infectious agents, in particular bacteria. Neutrophils contain granules filled with potent chemicals that can destroy bacteria or other nearby cells when the chemicals are released.

pathogens: disease-causing organisms such as bacteria, viruses and fungi.

pattern recognition receptors: molecules found on the surface of immune cells that help the body distinguish self from disease-causing pathogens. These include toll-like receptors.

receptors: molecules found on the surface of cells that bind to molecules on other cells, stimulating a variety of different functions.

synovium: membrane that lines the joints and secretes fluid that helps lubricate the joint. This membrane becomes inflamed in diseases such as rheumatoid arthritis, leading to symptoms such as pain, swelling, redness, and heat, and eventually to joint damage.

T cell: a type of lymphocyte. T cells have T cell receptors and, sometimes, co-stimulatory molecules on their cell surfaces. The T cell helps to orchestrate the immune system and can issue "orders" for other cells to make cytokines and chemokines.

T-cell receptor: a molecule found on the surface of T cells. The T-cell receptor can recognize and interact with a corresponding MHC molecule that is displaying an antigen.

thymus: a small organ that lies under the breastbone that makes white blood cells called lymphocytes that travel through the body and fight infection.

toll-like receptors: molecule found on the surface of immune cells such as dendritic cells, which recognize molecular patterns commonly seen in many bacteria and viruses but which are not present in humans, thereby allowing the immune system to detect foreign invaders.

trigger: something that either sets off a disease in people who are genetically predisposed to developing the disease, or that causes a certain symptom to occur in a person who has a disease. For example, sunlight can trigger rashes in people with lupus.