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The Body's Defenses


an enzyme in perspiration, tears, and saliva, that attacks bacterial cell walls


a type of endocytosis involving large, particulate substances


most abundant type of leukocyte. self-destruct as they destroy foreign invaders, limiting life span to a few days


an agranular leukocyte that is able to migrate into tissues and transform into a macrophage


an amoeboid cell that moves through tissue fibers, engulfing bacteria and dead cells by phagocytosis


white blood cell that are responsible for combating infection by parasites in vertebrates

natural killer (NK) cells

a nonspecific defensive cell that attacks tumor cells and destroys infected body cells, especially those harboring viruses

inflammatory response

a line of defense triggered by penetration of the skin or mucous membranes, in which small blood vessels in the vicinity of an injury dilate and become leakier, enhancing the inflitration of leukocytes; may also be widespread in the body


a substance released by injured cells that causes blood vessels to dilate during an inflammatory response


a circulating leukocyte that produces histamine

mast cell

a vertebrate body cell that produces histamine and other molecules that trigger the inflammatory response.


any of about 50 different proteins, secreted by many cell types near a site of injury or infection, that help direct migration of white blood cells to an injury site and induces other changes central to inflammation.


molecules that set the body's thermostat to a higher temperature; they are released by certain leukocytes.

complement system

group of at least 20 blood protiens that cooperate with other defense mechanisms; may amplify the inflammatory response, enhance phagocytosis, or directly lyse pathogens; activated by the onset of the immune response, or by surface antigens on microorganisms or other foreign cells.


a chemical messenger of the immune system, produced by virus-infected cells and capable of helping other cells resist the virus

B lymphocytes (B cells)

a type of lymphocyte that develops in the bone marrow and later produces antibodies, which mediate humoral immunity

T lymphocytes (T cells)

a type of lymphocyte responsible for cell-mediated immunity that differentiates under the influence of the thymus


a foreign macromolecule that does not belong to the host organism and that elicits an immune response.


an antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.

antigen receptors

transmembrane versions of antibody molecules that B cells and T cells use to recognize specific antigens; also called membrane antibodies

T cell receptors

antigen receptors on a T cell; unlike antibodies, T cell receptors are never produced in a secreted form

effector cells

a muscle cell or gland cell that performs the body's responses to stimuli; responds to signals from the brain or other processing center of the nervous system

memory cells

a clone of long-lived lymphocytes, formed during the primary immune response, which 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.

clonal selection

the mechanism that determines specificity and accounts for antigen memory in the immune system; occurs because an antigen introduced into the body selectively activates only a tiny fraction of inactive lymphocytes, which proliferate to form a clone of effector cells specific for the stimulating antigen.

primary immune response

the initial immune response to an antigen, which appears after a lag of several days

plasma cells

a derivative of B cells that secretes antibodies

secondary immune response

encounter of the same antigen at a different time; more rapid, of greater magnitude, longer than first time

major histocompatibility complex (MHC)

a large set of cell surface antigens encoded by a family of genes; foreign MHC markers trigger T-cell responses that may lead to the rejection of transplanted tissues and organs

Class I MHC molecules

a collection of cell surface glycoproteins encoded by a familiy of genes called the major histocompatibility complex. In humans, these glycoproteins are also known as HLA, human leukocyte antigens. Class I MHC molecules are found on all nucleated cells

Class II MHC molecules

a collection of cell surface glycoproteins encoded by a family of genes called the major histocompatibility compelx. In humans, these glycoproteins are also known as the HLA, human leukocyte antigens. Class II MHC molecules are restricted to a few specilaized cell types

antigen presentation

the process by which an MHC molecule cradles a fragment of an intracellular protein antigen in its hammocklike groove, carries it to the cell surface, and "presents" the protein to an antigen receptor on a nearby T cell.

cytotoxic T cells (Tc)

a type of lymphocyte that kills infected cells and cancer cells

helper T cells (Th)

a type of T cell that is required by some B cells to help them make antibodies or that helps other T cells respond to antigens or secrete lymphokines or interleukins

antigen-presenting cells (APCs)

cells that ingest bacteria and viruses and then destriy them

humoral immunity

the type of immunity that fights bacteia and viruses in body fluids wiht antibodies that circulate in blood plasma and lymph, fluids formerly called humors

cell-mediated immunity

the type of immunity that functions in defense against fungi, protists, bacteria, and viruses inside host cells and against tissue transplants, with highly specialized cells that circulate in the blood lymphoid tissue


a T cell surface protein, present on most helper T cells, CD4 binds to part of the class II MHC protein.


In the vertebrate immune system, protein factors secreted by macrophages and helper T cells as regulators of neighboring cells

interleukin-2 (IL-2)

a cytokine that helps B cells that have contacted antigen differentiate into antibody-secreting plasma cells

interleukin-1 (IL-1)

a cytokine secreted by a macrophage that is in the process of phagocytizing and presenting antigen


a T cell surface protein that enhances the interaction between the antigen-presenting infected cell and a cytotoxic T cell.


a protein secreted by a cytotoxic T cell that lyses an infected cell by perforating its membrane

tumor antigen

a foreign macromolecule, associated with a tumor, which does not belong to the host organism and that elicits an immune response.

T-dependent antigens

antigens that can stimulate antibody production only with help from T helper cells

T-independent antigens

antigen that can stimulate antibody production without the help of IL-2


a localized region on the surface of an antigen that is chemically recognized by antibodies; also called antigenic determinant

immunoglobulins (Igs)

one of the class of proteins comprising the antibodies

heavy chains

polypeptide chains that contribute to the structure of an antibody. Two identical heavy chains and two identical light chains, joined by disulfide brides, form a Y-shaped antibody molecule.

light chains

polypeptide chains that contribute to the structure of an antibody. Two identical light chains and two identical heavy chains, joined by disulfide bridges, form a Y-shaped antibody molecule.

monoclonal antibodies

a defensive protein produced by cells descended from a single cell; an antibody that is secreted by a clone of cells and, consequently, is specific for a single antigenic determinant


an immune response in which an antibody binds to and blocks the activity of an antigen.


an immune response in which the binding of antibodies to the surface of a microbe facilitates phagocytosis of the the microbe by a macrophage


an antibody-mediated immune response in which bacteria or viruses are clumped together, effectively neutralized, and opsonized.

complement fixation

an immune response in which antigen-antibody complexes activate complement proteins

membrane attack complex (MAC)

a molecular complex including complement proteins that generates a 7-10-nm diameter pore in a bacterial membrane, causing the cell to die

immune adherence

the collective action of antibodies, complement, and phagocytes. Microbes coated with antibodies and complement proteins adhere to blood vessel walls, making the pathogens easier prey for phagocytic cells circulating in the blood

active immunity

immunity conferred by recovering from an infectious disease


a process of stimulates the body's immune system to defend against attack by particular contagious disease. a person may acquire these either naturally (by having the disease) or thorugh vaccination (by having an injection, wearing a patch swallowing, or inhaling)


an injection into the body that contains antigens characteristic of a particular disease organism and the stimulates an immune response

passive immunity

temporary immunity obtained by acquiring ready-made antibodies or immune cells; lasts only a few weeks or months because the immune system has not been stimulated by antigens

ABO blood groups

genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of red blood cells; phenotypes, also called blood types, are A, B, AB, and O

Rh factor

a category of erythrocyte antigen that generates antibodies of the IgG class

graft versus host reaction

an attack against a patient's body cells by lymphocytes received in a bone marrow transplant

anaphylactic shock

an acute, life-threatening, allergic response

acquired immunodeficiency syndrome (AIDS)

a syndrom caused by the human immunodeficiency virus HIV that renders immune cells ineffective permitting opportunistic infections malignancies and neurological diseases to develop it is transmitted sexually or through exposure to contaminated blood

human immunodeficiency virus (HIV)

a sexually transmitted virus that destroys white blood cells in the immune system, leaving the body vulnerable to life-threatening diseases.

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