The Immune System

ABO blood groups

Genetically determined classes of human blood; based on the presence or absence of carbohydrates A and B on the surface of red blood cells. The ABO blood group phenotypes, also called blood types, are A, B, AB, and O.

acquired immunity

The kind of defense that is mediated by B lymphocytes (B cells) and T lymphocytes (T cells). It exhibits specificity, memory, and self-nonself recognition. Also called adaptive immunity.

active immunity

Long-lasting immunity conferred by the action of a person's B cells and T cells and the resulting B and T memory cells specific for a pathogn. Active immunity can develop as a result of natural infection or immunization.

AIDS (acquired immunodeficiency syndrome)

The name of the late stages of HIV infection, defined by a specified reduction of T cells and the appearance of characteristic secondary infections.

anaphylactic shock

An acute, whole-body, life-threatening, allergic response.

antibody

A protein secreted by plasma cells (differentiated B cells) that binds to a particular antigen and marks it for elimination; also called immunoglobulin. All antibody molecules have the same Y-shaped structure and in their monomer form consist of two identical heavy chains and two identical light chains joined by disulfide bridges.

antigen

A macromolecule that elicits an immune response by lymphocytes.

antigen presentation

The process by which an MHC molecule binds to a fragment of an intracellular protein antigen and carries it to the cell surface, where it is displayed and can be recognized by a T cell.

antigen receptor

The general term for a surface protein, located on B cells and T cells, that binds to antigens, initiating acquired immune responses. The antigen receptors on B cells are called B cell receptors (or membrane immunoglobulins), and the antigen receptors on T cells are called T cell receptors.

antigen-presenting cell

A cell that ingests bacteria and viruses and destroys them, generating peptide fragments that are bound by class II MHC molecules and subsequently displayed on the cell surface to helper T cells. Macrophages, dendritic cells, and B cells are the primary antigen-presenting cells.

apoptosis

The changes that occur within a cell as it undergoes programmed cell death, which is brought about by signals that trigger the activation of a cascade of suicide proteins in the cell destined to die.

autoimmune disease

An immunological disorder in which the immune system turns against self.

B cell receptor

The antigen receptor on B cells: a Y-shaped, membrane-bound molecule consisting of two identical heavy chains and two identical light chains linked by disulfide bridges and containing two antigen-binding sites; also called a membrane immunoglobulin or membrane antibody.

B lymphocyte (B cell)

A type of lymphocyte that develops to maturity in the bone marrow. After encountering antigen, B cells differentiate into antibody-secreting plasma cells, the effector cells of humoral immunity.

CD4

A surface protein, present on most helper T cells, that binds to class II MHC molecules on antigen-presenting cells, enhancing the interaction between the T cell and the antigen-presenting cell.

CD8

A surface protein, present on most cytotoxic cells, that binds to class I MHC molecules on target cells, enhancing the interaction between the T cell and the target cell.

cell-mediated immune response

The branch of acquired immunity that involves the activation of cytotoxic T cells, which defend against infected cells, cancer cells, and transplanted cells.

class I MHC molecules

A collection of cell surface proteins encoded by a family of genes called the major histocompatibility complex. Class I MHC molecules are found on nearly all nucleated cells.

class II MHC molecules

A collection of cell surface proteins encoded by a family of genes called the major histocompatibility complex. Class II MHC molecules are restricted to a few specialized cell cell types, commonly called antigen-presenting cells (dendritic cells, macrophages, and B cells).

clonal selection

The process by which an antigen selectively binds to and activates only those lymphocytes bearing receptors specific for the antigen. The selected lymphocytes proliferate and differentiate into a clone of effector cells and a clone of memory cells specific for the stimulating antigen. Clonal selection accounts for the specificity and memory of acquired immune responses.

complement system

A group of proteins that complement the antibodies and phagocytic cells; assists in ridding an organism of microbes and damaged cells; promotes inflammation.

cytokine

Any of a group of proteins secreted by a number of cell types, including macrophages and helper T cells, that regulate the function of lymphocytes and other cells of the immune system.

epitope

A small, accessible region of an antigen to which an antigen receptor or antibody binds; also called an antigenic determinant.

helper T cell

A type of T cell that, when activated, secretes cytokines that promote the response of B cells (humoral response) and cytotonic T cells (cell-mediated response) to antigens.

histamine

A substance released by mast cells that causes blood vessels to dilate and become more permeable during an inflammatory response.

HIV (human immunodeficiency virus)

The infectious agent that causes AIDS. HIV is a retrovirus.

humoral immune response

The branch of acquired immunity that involves the activation of B cells and that leads to the production of antibodies, which defend against bacteria and viruses in body fluids.

immunization

The process of generating a state of immunity by artifical means. Active immunization: nonpathogenic version of a normally pathogenic microbe is administered, inducing B and T cell responses and immunological memory. Passive immunization: antibodies specific for a particular microbe are administered, conferring immediate but temporary protection.

immunoglobulin (Ig)

Any of the class of proteins that function as antibodies. Immunoglobulins are divided into five major classes that differ in their distribution in the body and antigen disposal activities.

inflammatory response

A localized innate immune defense triggered by physical injury or infection of tissue in which changes to nearby small blood vessels enhance the infiltration of white blood cells, antimicrobial proteins, and clotting elements that aid in tissue repair and destruction of invading pathogens; may also involve systemic effects such as fever and increased production of white blood cells.

innate immunity

The kind of defense that is mediated by phagocytic cells, antimicrobial proteins, the inflammatory response, and natural killer (NK) cells. It is present before exposure to pathogens and is effective from the time of birth.

interferon

A protein that has antiviral or immune regulatory functions; secreted by virus-infected cells to help nearby cells resist viral infection

lymphatic system

A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid, proteins, and cells to the blood.

lymphocyte

A type of white blood cell that mediates acquired immunity. Lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.

lysozyme

An enzyme in sweat, tears, and saliva that attacks bacterial cell walls.

macrophage

A phagocytic cell present in many tissues that functions in innate immunity by destroying microbes and in acquired immunity as an antigen-presenting cell.

major histocompatibility complex (MHC)

A family of genes that encode a large set of cell surface proteins called MHC molecules. Class I and class II MHC molecules function in antigen presentation to T cells. Foreign MHC molecules on transplanted tissue can trigger T cell responses that may lead to rejection of the transplant.

memory cell

One of a clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymphoid organ until activated by exposure to the same antigen that triggered its formation. Activated memory cells mount the secondary immune response.

natural killer (NK) cell

A type of white blood cell that can kill tumor cells and virus-infected cells; an important component of innate immunity.

neutrophil

The most abundant type of white blood cell. Neutrophils are phagocytic and tend to self-destruct as they destroy foreign invaders, limiting their life span to a few days.

passive immunity

Short-term immunity conferred by the administration of ready-made antibodies or the transfer of maternal antibodies to a fetus or nursing infant; lasts only a few weeks or months because the immune system has not been stimulated by antigens.

phagocytosis

A type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendritic cells.

plasma cell

The antibody-secreting effector cell of humoral immunity; arises from antigen-stimulated B cells.

primary immune response

The initial acquired immune response to an antigen, which appears after a lag of about 10 to 17 days.

Rh factor

A protein antigen on the surface of red blood cells designated Rh-positive. If an Rh-negative mother is exposed to blood from an Rh-positive fetus, she produces anti-Rh antibodies of the IgG class.

secondary immune response

The acquired immune response elicited on second or subsequent exposures to a particular antigen. The secondary immune response is more rapid, of greater magnitude, and of longer duration than the primary immune response.

T cell receptor

The antigen receptor on T cells; a membrane-bound molecule consisting of one ? chain and one ? chain linked by a disulfide bridge and containing one antigen-binding site.

T lymphocyte (T cell)

A type of lymphocyte, including the helper T cells and cytotoxic T cells, that develops to maturity in the thymus. After encountering antigen, T cells are responsible for cell-mediated immunity.

thymus

A small organ in the thoracic cavity of vertebrates where maturation of T cells is completed.