(Italicized terms are also deﬁned within this glossary or the glossary of Selected Diseases Related to the Immune System.)
active immunity: usually long-lasting immunity that is acquired through the production of antibodies and memory T cells within the organism in response to the presence of antigens.
adaptive immune system: also called the acquired immune system, this component of the immune system comprises white blood cells, particularly lymphocytes. When it is presented with a new microbe or vaccine, it may take days or weeks to respond or adapt, but the resultant improved immune readiness, or “memory,” is sustained for long periods (years).
adenosine deaminase (ADA): an enzyme found in mammalian tissues that is capable of catalyzing the process in which adenosine is split into inosine and ammonia. A deﬁciency can cause problems with metabolic reactions in cells, which leads to the destruction of B and T cells. ADA deﬁciency can lead to one form of severe combined immunodeﬁciency disease.
allergy: a misguided reaction by the immune system to harmless foreign substances.
antibody: a protein on the surface of B cells that is also secreted in large amounts into the blood or lymph in response to an antigen, a component within an invader such as a bacterium, virus, parasite, or transplanted organ. Antibodies neutralize the antigen, and thereby the invader, by binding to it, often directing it toward a macrophage for destruction. Also called an immunoglobulin.
antigen: a foreign substance (usually a protein or carbohydrate) capable of triggering an immune response in an organism.
antiretroviral drugs: drugs that act against retroviruses (such as HIV ).
autoimmune disorders: conditions in which the body’s own immune system acts against it.
autoreactive: describes immune cells that mount a response against the body’s own cells or tissues.
B cell: a type of lymphocyte that produces antibodies, which bind to free-ﬂoating microbes circulating in the blood so that they cannot infect other cells.
biochemicals: chemicals produced within living organisms. Many coordinate to ﬁght off invasion in an immune response.
biological barriers: the body’s ﬁrst layer of protection against harmful microbes; skin is a prime example.
blood-brain barrier: a tight layer of cells and tissue that separates the brain from the rest of the body; a physical roadblock that normally keeps immune cells outside the brain.
blood-forming stem cells: immature cells in the bone marrow that multiply extensively and produce red and white blood cells and platelets.
CD4+ helper T cells:T cells with CD4 receptors that respond to antigens on the surface of speciﬁc molecules by secreting a certain type of cytokine that stimulates B cells and killer T cells. Helper T cells are infected and killed by HIV; people who develop AIDS have no more than one-ﬁfth the normal number of helper T cells.
central nervous system: the brain and spinal cord, to which sensory impulses are transmitted and from which motor impulses emanate. The central nervous system supervises and coordinates the activity of the entire nervous system and interacts with the immune system.
clones: copies that viruses make of themselves.
cytokine: a class of substance secreted by cells of the immune system to regulate immune cells.
dendritic cell: an antigen-presenting immune cell that initiates the immune response by activating lymphocytes and stimulating the secretion of cytokines. Dendritic cells also prevent autoimmune reactions by instructing the T lymphocytes to be silent or tolerant to the body itself.
DNA (deoxyribonucleic acid): nucleic acid that carries the cell’s genetic information and is capable of self-replication and the synthesis of RNA.
DNA vaccine: vaccines that often use “naked” DNA (DNA not associated with a cell or a virus) with instructions for making protective antigens. When injected, the DNA is taken in by other cells, which then produce protective antigens.
E. coli: a bacterium that is used in public health as an indicator of fecal pollution (as of water or food) and in medicine and genetics as a research organism. E. coli occurs in various strains that may live as harmless inhabitants of the human lower intestine or may produce a toxin causing intestinal illness.
enzymes: complex proteins produced by living cells and that catalyze speciﬁc reactions from biochemicals.
epidemic: an outbreak of disease that simultaneously affects an atypically large number of individuals within a population, community, or region.
estrogen: a steroid hormone produced chieﬂy by the ovaries, responsible for promoting development and maintenance of female secondary sex characteristics. Estrogen may play a role in certain immune system diseases.
genetic engineering: deliberate alteration of genetic material by intervention in genetic processes.
granulocyte: a type of phagocyte with cytoplasm that contains grainlike particles.
highly active antiretroviral therapy (HAART): a treatment to combat AIDS using several antiretroviral drugs at the same time.
histamine: a compound found in mammalian tissues that causes stretching of capillaries, contraction of smooth muscle, and stimulation of gastric acid secretions; released during allergic reactions.
immune deﬁciency diseases (IDDs): diseases that result when one or more parts of the immune system are missing or defective.
immunoglobulin E (IgE): a class of antibodies that function in allergic reactions.
immunosuppressive: describes a treatment that suppresses natural immune responses—for example, chemotherapy for cancer.
inactivated vaccines:vaccines made by growing and purifying large numbers of the target organism in the laboratory and then killing them with heat, radiation, or chemicals. The immune system reacts to the dead microorganisms, producing immunity.
inﬂammation: a buildup of ﬂuid and cells that occurs as the immune system ﬁghts a hostile invader.
innate immune system: component of the immune system that consists of a set of genetically encoded responses to pathogens and does not change or adapt during the lifetime of the organism. Innate immunity involves quickly mobilized defenses triggered by receptors that recognize a broad spectrum of microbes; in contrast to adaptive immunity, it does not acquire memory for an improved response during a second exposure to infection.
killer T cell: a type of lymphocyte that directly attacks and kills infected cells or other targets, including tumor cells and even one’s own tissues. Killer cells are generated by the coordinated action of dendritic cells and CD4+ helper T cells.
knockout: term used in genetic engineering when a speciﬁc gene is deliberately removed in order to create an organism unable to carry out the functions the gene codes for; knockouts are used by immunologists to determine the functions of speciﬁc genes that encode immune proteins.
latency: the state or period in which a virus has invaded a host but is not actively multiplying, and during which symptoms of the infection are not seen. “Microbial latency” means the microbe is not multiplying, as occurs in some cells in HIV infection, while “clinical latency” means that the patient does not have symptoms of disease even though the virus is multiplying and damaging the immune system. In HIV, clinical latency precedes the AIDS stage.
lymph nodes: small, rounded structures in the lymphatic system that contain disease-ﬁghting white blood cells, especially lymphocytes, and ﬁlter out harmful microbes and toxins. Lymph nodes may become enlarged when they are actively ﬁghting infection.
lymphocyte: a type of white blood cell involved in the human body’s immune system, of which there are two broad categories, T cells and B cells. Lymphocytes are an integral part of the body’s defenses because they are highly speciﬁc for antigens associated with microbes, tumor cells, transplants, allergies, and tissues attacked in autoimmune diseases. The immune system comprises clones of lymphocytes, each with a single speciﬁcity, and exposure to antigens leads to clonal expansion, the acquisition of helper and killer functions, and formation of immune memory.
lysozyme: an enzyme in saliva and tears that destroys bacteria.
macrophages: large phagocyte cells that remove harmful microbes from the body.
major histocompatibility complex (MHC) molecules: a group of molecules that help the immune system distinguish between harmful and safe foreign substances in the body. Recent research suggests some classes of MHC molecules also play an essential role in brain function.
mast cells: large cells, found in connective tissues, that mediate allergic reactions. Mast cells play an important protective role in wound healing and defend against pathogens.
memory B and T cells:B and T cells that remain in the body after the completion of an immune response to ward off future attacks by the same microbe. Memory is imparted by the increased size in the antigen-speciﬁc B or T cell clone, as well as improved function of individual cells within the clone.
microglia: specialized immune cells, related to macrophages, that protect the central nervous system.
molecular mimicry: an occurrence in many autoimmune disorders in which a microbe carries antigens that resemble those on a particular organ, causing the immune system to attack the body.
monoclonal antibodies: antibodies derived from a single cell and used against a speciﬁc antigen such as a cancer cell. Rituxan and Herceptin are monoclonal antibodies used in the treatment of lymphoma and breast cancer, respectively.
mutation: a process in which a microbe or organism undergoes a permanent change in hereditary material. When viruses or bacteria mutate they are no longer recognized by the immune system and become resistant to previously administered vaccines and drugs.
myelin: a white, fatty material that sheathes nerves and enhances the transmission of signals between the brain and the body. In multiple sclerosis, an autoimmune disorder, immune cells attack myelin, affecting the transmission of nerve signals.
pandemic: an outbreak of disease occurring over a wide geographical area and affecting an exceptionally high proportion of the population.
passive immunity: immunity acquired by the transfer of antibodies (as by injection of serum from an individual with active immunity).
pathogen: a speciﬁc causative agent of disease, such as a bacterium or a virus.
penicillin: a mixture of nontoxic antibiotics produced by mold and used regularly to treat harmful bacteria.
phagocyte: a cell such as a white blood cell that engulfs and consumes foreign material, such as microorganisms.
plasma cell: an antibody-producing lymphocyte derived from a B cell upon reaction with a speciﬁc antigen.
protease: an enzyme that catalyzes the splitting of proteins into smaller molecules. To treat AIDS, scientists have designed drugs that interfere with protease made by the HIV virus, which is essential to its replication.
reassortment: the constant state of ﬂux and rearrangement seen in the genes of viruses.
red blood cells: any of the hemoglobin-containing cells that carry oxygen to the tissues and are responsible for the red color of vertebrate blood..
regulatory T cells (Treg cells): special T cells that regulate or suppress immune responses, preventing autoimmunity for example.
replication: process by which an organism produces a copy of itself—for example, the way microbes reproduce.
respiratory syncytial virus (RSV): a virus that forms masses, or syncytia, in tissue culture and that is responsible for severe respiratory diseases.
retrovirus: a type of RNA virus (such as HIV ) that reproduces by transcribing itself into DNA (using reverse transcriptase). The resultant DNA inserts itself into a cell’s DNA and is reproduced by the cell.
reverse transcriptase (RT): an enzyme that catalyzes the formation of DNA using RNA as a template.
RNA (ribonucleic acid): a group of molecules similar in structure to a single strand of DNA. The function of RNA is to carry the information from the DNA in the cell’s nucleus into the body of the cell to assemble proteins.
rotavirus: a retrovirus with a double-layer protein shell and a wheel-like appearance. Rotaviruses cause diarrhea, especially in infants.
stem cell transplants: a kind of passive immune therapy that transfers cells instead of antibodies. Stem cells have the capacity to give rise to all elements of the immune system, such as many types of lymphocytes and phagocytes.
steroids: a large family of chemical substances, comprising many hormones, body constituents, and drugs; they are often immunosuppressive.
subunit vaccines:vaccines that contain only a part of the target microorganism.
synapses: specialized junctions at which cells of the nervous system signal to one another and to nonneuronal cells, such as those of muscles and glands.
T cell: a type of lymphocyte that possesses highly speciﬁc cell-surface antigen receptors; types include CD4+ helper T cells, regulatory T cells, and killer T cells.
tolerance: the capacity of the body to become less responsive to a substance or a physiological insult. Tolerance to components of the self prevents or suppresses autoimmunity.
toxoid vaccine: an inactivated and weakened version of the disease-causing toxin a microbe produces; it is still capable of inducing the formation of antibodies when injected.
transgenic technology: technology used to deliberately alter the genome of an organism by the transfer of a gene or genes from another species or breed.
two-photon microscopy: an imaging technique using high-powered laser microscopes to examine immune response in the nervous system.
vaccine: killed microorganisms, weakened living organisms, fully virulent living organisms, or subunit proteins of a microbe, administered to produce or artiﬁcially increase immunity to a particular disease.
vector vaccines: vaccines made by inserting protective antigen genes into harmless bacteria or viruses (vectors). As the vectors multiply in the body, they expose the immune system to protective antigens, stimulating active immunity against the harmful organism.
white blood cells: any of the blood cells that are colorless, lack hemoglobin, and contain a nucleus. They include the lymphocytes, dendritic cells, monocytes, neutrophils, eocinophils, and basophils; also called leukocytes.