Immune System

Active immunity

long lasting immunity conferred by the action of B cells and T cells and resulting B and T memory cells specific for a pathogen. Results from natural infection or immunization

Adaptive immunity

A vertebrate specific defense that is mediated by B lymphocytes and T lymphocytes that exhibits specificity, memory and self-nonself recognition

Antibody

A protein secreted by plasma cells that binds to a particular antigen

antigen (an′-ti-jen)

A substance that elicits an immune response by binding to receptors of B or T cells.

B cells

The lymphocytes that complete their development in the bone marrow and become effector cells for the humoral immune response

cell-mediated immune response

The branch of adaptive immunity that involves the activation of cytotoxic T cells, which defend against infected cells

clonal selection

The process where an antigen selectively binds to and activates only those lymphocytes with receptors for specific antigens and they proliferate and differentiate into clones

complement system

a group of blood proteins that amplify the inflammatory response, enhance phagocytosis or directly lyse extracellular pathogens

cytotoxic T cells

A type of lymphocyte that, when activated, kills infected cells as well as certain cancer cells and transplanted cells.

Dendritic cells

An antigen-presenting cell that is particularly efficient in presenting antigens to helper T cells, thereby initiating a primary immune response

epitope

A small, accessible region of an antigen to which an antigen receptor or antibody binds.

helper T cell

A type of T cell that, when activated, secretes cytokines that promote the response of B cells and cytotoxic T cells to antigens

histamine

A substance released by mast cells that causes blood vessels to dilate and become more permeable in inflammatory and allergic responses

humoral immune response (hyū′-mer-ul)

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

immunoglobulin

A protein secreted by plasma cells that binds to a particular antigen

inflammatory response

An innate immune defense involving the release of substances that promote swelling, white blood cells, and tissue repair

innate immunity

A form of defense common to all animals that is active immediately upon exposure to a pathogen

interferons

A protein that has antiviral or immune regulatory functions by helping nearby cells resist viral infection or helping to activate macrophages.

lymphocytes

A type of white blood cell that mediates immune responses. The two main classes are B cells and T cells

macrophages

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

A host protein that functions in antigen presentation and can trigger T cell responses that may lead to rejection of a transplant.

mast cells

A vertebrate body cell that produces histamine and other molecules that trigger inflammation in response to infection and in allergic reactions.

memory cells

One of a clone of long-lived lymphocytes, that remains in a lymphoid organ until activated by exposure to the same antigen that triggered its formation.

monoclonal antibodies

Any of a preparation of antibodies that have been produced by a single clone of cultured cells and thus are all specific for the same epitope.

natural killer cells

A type of white blood cell that can kill tumor cells and virus-infected cells as part of innate immunity

neutrophils

The most abundant type of white blood cell that 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 transfer of antibodies, as occurs in the transfer of maternal antibodies to a fetus or nursing infant.

plasma cells

The antibody-secreting effector cell of humoral immunity that arise from antigen-stimulated B cells.

primary immune response

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

secondary immune response

The rapid adaptive immune response elicited on second or subsequent exposures to a particular antigen that is of greater magnitude, and of longer duration.

T cells

The class of lymphocytes that include both effector cells for the cell-mediated immune response and helper cells required for both branches of adaptive immunity.

Thymus

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