Resistant to infection
Immune
The tissues, cells, and molecules involved in the defense of the body against infectious agents.
Immune system
The ability to resist a specific infection
Immunity
The deliberate induction of protective immunity to a pathogen by the administration of killed or non-pathogenic forms of the pathogen, or its antigens, to induce an immune response.
Vaccination
The deliberate provocation of an adaptive immune response by introducing antigen into the body.
Immunization
A microorganism that habitually lives on or in the human body; one that normally causes no disease or harm and can be beneficial.
Commensal
The microorganisms that habitually live in or on the human body; they normally do not cause disease and in many cases provide positive benefit.
microbiota
An organism, most commonly a microorganism, that can cause disease.
Pathogen
A microorganisms that causes disease only in individuals whose immune systems are in some way compromised.
Opportunistic pathogens
Diverse prokaryotic microorganisms that are responsible for many infectious diseases of humans and other animals. Some bacterial pathogens live only extracellularly, colonizing tissue surfaces and intercellular spaces; others can invade cells and live intracellularly.
Bacteria (singular bacterium)
Submicroscopic pathogen composed of a nucleic acid genome enclosed in a protein coat. ( ___ ) replicate only inside living cells bacause they do not possess all the metabolic machinery required for independent life. A viral particle is called a virion.
Virus(es)
A group of single-celled and muticellular eukaryotic organisms, including the yeasts and molds, that can cause a variety of disease. Immunity to ( ___ ) involves both antibody-mediated and cell-mediated responses.
Fungi (singular fungus)
General name for the unicellular protozoa and muticellular worms that infect animals and humans and live within them, causing disease.
Parasite
General name for a type of supracellular structure composed of a single layer (for example the lining of the gut and respiratory tract) or multiple layers (for example the epidermis) of cells bound tightly to each other. They are generally found as surface layters that communicate between the rest of a tissue and its environment. ( ___ ) line the internal and external cavities of the body, and are also part of many internal organs.
Epithelium (plural epithelia)
Mucus-secreting epithelium such as that lining the respiratory, intestinal, and urogenital tracts. The mammary glands and the conjunctiva of the eye are also considered in this category. Mucosal epithelium communicates with the external environment and is the route of entry of most pathogens.
Mucosa (plural mucosae)
Slimy protective secretion composed of glycoproteins, proteoglycans, peptides, and enzymes that is produced by the goblet cells in many internal epithelia.
Mucus
Immune response that is initiated immediately on infection and does not depend on lymphocytes. It depends on host defenses such as complement, neutrophils, macrophages, and NK cells, which provide nonspecific defense against a wide range of pathogens. This response does not generate immunological memory.
Innate immune response
Any of the physiological and cellular processes used by the immune system to destroy pathogens and remove them from the body.
Effector mechanisms
A terminally differentiated activated lymphocyte that can kill pathogens or remove them from the body without the need for further differentiation.
Effector cells
Collection of plasma proteins that act in a cascade of reactions to attack extracellular forms of pathogens in extracellular spaces and the blood. Pathogens become coated with ( ___ ) proteins, which can either kill the pathogen directly or facilitate its engulfment and destruction by phagocytes. It is involved in both innate and adaptive immunity and is activated either directly or indirectly by the presence of infection.
Complement
The host defense mechanisms that act from the start of an infection and do not adapt to a particular pathogen or generate immunological memory.
Innate immunity
Any of a large number of proteins secreted by cells that act locally to change the behavior of neighboring cells. ( ___ ) act by binding to specific receptors on their target cells. ( ___ ) made by lymphocytes are often called lymphokines or interleukins (abbreviated IL).
Cytokine(s)
General term for the local accumulation of fluid, plasma proteins, and white blood cells that is initiated by physical injury, infection, or a local immune response. It is also known as an inflammatory response. The cells that invade tissues and help mediate ( ___ ) are often called inflammatory cells. Those cytokines that promote ( ___ ) are known as inflammatory cytokines.
Inflammation
Epithelium lining the interior of blood vessels.
Endothelium
Abnormal accumulation of fluid in connective tissue, leading to swelling.
Edema
General name for a class of white blood cells consisting of three main cell types. B ( ___ ) (B cells) and T ( ___ ) (T cells) are small cells that bear variable cell-surface receptors for antigen and are responsible for adaptive immune responses. Natural killer (NK) cells are large grannular cells and are the ( ___ ) of innate immunity.
Lymphocyte(s)
The response of antigen-specific B and T lymphocytes to antigen, including the development of immunological memory.
Adaptive Immune response
The state of resistance to infection that is produced by the adaptive immune response.
Adaptive immunity
The central principle of adaptive immunity. It is the mechanisms by which adaptive immune responses derive only from individual antigen-specific lymphocytes, which are stimulated by the antigen to proliferate and differentiate into antigen-specific effector cells.
Clonal selection
The multiplication of lymphocytes after their activation by antigen, so that large clones of rare antigen-specific lymphocytes are generated to figth the infecting pathogen.
Clonal expansion
The capacity of the immune system to make quicker and stronger adaptive immune responses to successive encounters with an antigen. It is specific for a particular antigen and is long-lived.
Immunological memory
Alternative term for adaptive immunity; pathogen-specific immunity acquired as a consequence of infection or vaccination.
Acquired immunity
The specific immunological resistance to a pathogen that is present in an individual during months after either vaccination or recovery from an infection with the pathogen, and which is due to pathogen-specific antibodies and effector T cells produced during the primary response.
Protective immunity
The adaptive immune response that follows a person's first exposure to an antigen.
Primary immune response
The adaptive immune response provoked by a second exposure to an antigen. It differs from the primary response by starting sooner and building more quickly, and is due to the presence of long-lived memory B cells and T cells specific for the antigen.
Secondary immune response
General term for a white blood cell. Lymphocytes, granulocytes, and monocytes are all ( ___ ).
Leukocyte(s)
The generation of the cellular elements of blood, including the red blood cells, white blood cells, and platelets. These cells all originate from pluripotent hematopoietic stem cells whose differentiated progeny divide under the influence of various hematopoietic growth factors.
Hematopoiesis
The stem cell in bone marrow that gives rise to all the cellular elements of the blood.
Pluripotent hematopoietic stem cell
Red blood cell.
Erythrocyte
Large cell of the erythroid lineage, produced in the bone marrow and resident there, from which blood platelets are produced by fragmentation.
Megakaryocyte
A small non-nucleated cell fragment, derived from megakaryocytes, that is present in large numbers in the blood and is involved in blood clotting.
Platelet
Any blood cell or its precursor cell types.
Hematopoietic cells
Tissue in the center of certain bones that is the major site of generation of all the cellular elements of the blood (hematopoiesis).
Bone marrow
The ability of a population of cells, such as stem cells, to maintain itself permanently by multiplication and limited differentiation.
Self renewal
The stem cell in bone marrow that gives rise to the myeloid lineage (granulocytes, monocytes, macrophages, mast cells, and dendritic cells).
Myeloid progenitor
The granulocytes, monocytes, macrophages, mast cells and dendritic cells, and the bone maroow cells that give rise to them.
Myeloid lineage
White blood cell with irregularly shaped, multilobed nuclei, and cytoplasmic granules. There are three types of granulocyte; neutrophil, eosinophil, and basophil. ( ___ ) are also known as polymorphonuclear leukocytes.
Granulocyte(s)
An alternative name for a granulocyte (a class of white blood cells comprising neutrophils, eosinophils, and basophils), describing the varied morphology of the nuclei.
Polymorphonuclear leukocytes
Phagocytic white blood cell that enters infected tissues in large numbers and engulfs and kills extracellular pathogens. It is a type of granulocyte and contains granules that stain with neutral dyes; hence its name. It is by far the most abundant white blood cell.
Neutrophil
A cell specialized to perform phagocytosis. The principal phagocytic cells in mammals are neutrophils and macrophages.
Phagocyte
Thick yellowish-white fluid that is formed in infected wounds. It is composed of dead and dying white blood cells (principally neutrophils), tissue debris, and dead microorganisms.
Pus
White blood cell that is one of the three types of granulocytes. It contains granules that stain with eosin (hence their name) and whose contents are secreted when the cell is stimulated. ( ___ ) contribute chiefly to defense against parasitic infections.
Eosinophil(s)
White blood cell present in small numbers in the blood; it is one of the three types of granulocyte. ( ___ ) contain granules that stain with basic dyes; hence their name.
Basophil(s)
Phagocytic white blood cell with a bean-shaped nucleus. It is the precursor of the tissue macrophage.
Monocyte
Large mononuclear phagocytic cell resident in most tissues that bears receptors for many components of pathogens. ( ___ ) are derived from blood monocytes and contribute to innate immunity and early nonadaptive phases of host defense, as well as being effector cells in adaptive immune responses. They function as professional antigen-presenting cells and as phagocytic scavenger cells, and produce cytokines that recruit a variety of immune-system cells to an ongoing immune response.
Macrophage(s)
Professional antigen-presenting cell with a branched, dendrite-like morphology that are present in tissues. It is derived from the bone marrow and is distinct from the follicular dendritic cell that presents antigen to B cells. Immature dendritic cells take up and process antigens but cannot yet stimulate T cells. Mature or activated dendritic cells are present in secondary lymphoid tissues and are able to stimulate T cells.
Dendritic cells
Large bone marrow derived cell resident in connective tissues throughout the body. ( ___ ) contain large granules that store a variety of chemical mediators including histamine. ( ___ ) have high-affinity Fcε receptors (FcεRI) that bind free IgE. Antigen binding to IgE associated with ( ___ ) triggers mast-cell activation and degranulation, producing a local or systemic immediate hypersensitivity reaction. ( ___ ) have a crucial role in allergic reactions.
Mast cell(s)
Stem cell in bone marrow cell that gives rise to all lymphocytes.
Lymphoid progenitor
All types of lymphocyte, and the bone marrow cells that give rise to them.
Lymphoid lineage
An alternative and earlier name for the natural killer cell (NK cell). Microscopy revealed two types of blood lymphocyte: the small lymphocytes (B cells and T cells) and the ( ___ ) (NK cells).
Large granular lymphocytes
Large, granular, cytotoxic lymphocyte that circulates in the blood and is important in innate immunity to intracllular pathogens such as viruses. ( ___ ) do not have variable receptors for antigen but have numerous other receptors by which they can recognize and kill virus-infected cells and certain tumor cells. Also known as large granular lymphocyte.
Natural killer cells (NK cells)
The general name for a recirculating resting T or B lymphocyte.
Small lymphocytes
Alternative name for B cell.
B lymphocytes (B cells)
Alternative name for T cell.
T lymphocytes (T cells)
The general name for antibodies and B-cell antigen receptors.
Immunoglobulins
The highly variable antigen receptor of T lymphocytes. On most circulating T cells it is composed of a variable α chain and a variable β chain and is known as the α:β ( ___ ). This receptor recognizes peptide antigens derived from the breakdown of proteins. A minority of circulating T cells carry a γ:δ ( ___ ), in which the two chains are called γ and δ and are generally less diverse in their specificity and can recognize antigens other than peptides, such as phosphoantigens and glycolipids. Both types fo receptor are present at the cell surface in association with a complex of invariant CD3 chains and ζ chains, which have signaling function.
T-cell receptor(s)
Terminally differentiated B lymphocyte that secretes antibody.
Plasma cells
The secreted form of the immunoglobulin expressed by a B cell. It is produced by the plasma cells that differentiate from B cells.
Antibody
Any molecule or molecular fragment that either is recognized by an antibody or B-cell receptor or can be bound by an MHC molecule and presented to a T-cell receptor.
Antigen
The highly variable cell-surface receptor on lymphocytes that recognizes antigen. For a B cell, the receptor is a cell-surface immunoglobulin; for a T cell, the receptor is a rather similar molecule called the T-cell receptor. All the ( ___ ) on an individual lymphocyte are identical and recognize the same antigen epitope.
Antigen receptor(s)
Type of effector T cell that kills its target cells. It expresses the CD8 co-receptor and recognizes peptide antigens presented by MHC class I molecules. It is important in host defense against viruses and other cytosolic pathogens because it can recognize and kill the infected cells.
cytotoxic T cells
General name for effector CD4 T cells that function to help other immune-system cells perform their roles, such as to help B cells produce antibody.
Helper T cells
An antigen-specific CD4 T cell that functions to suppress of limit immune responses. Also called suppressor T cell.
Regulatory T cells (Treg)
Immunity that is mediated by antibodies and can therefore be transferred to a non-immune recipient by serum.
Humoral immunity
Old-fashioned term for the body fluids.
Humors
The mechanism by which antibodies binding to sites on pathogens prevent growth of the pathogen and/or its entry into cells. The toxicity of bacterial toxins can similarly be neutralized by bound antibody.
Neutralization
The coating of the surface of a pathogen or other particle with any molecule that makes it more readily ingested by phagocytes. Antibody and complement opsonize extracellular bacteria for phagocytosis by neutrophils and macrophages because the phagocytic cells carry receptors for these molecules.
Opsonization
Tissue that contain large numbers of lymphocytes, including the permanent organized lymphoid organs and more diffuse and transient collections of lymphocytes that arise in mucosal tissues.
Lymphoid tissues
Any of numerous organized tissues that contain large numbers of lymphocytes held in a non-lymphoid stroma. The primary lymphoid organs, where lymphocytes are generated, are the thymus and bone marrow. The main secondary lymphoid tissues, in which adaptive immune resposnes are initiated, are the lymph nodes, spleen, and mucosa-associated lymphoid tissues such as tonsils, Peyer's patches, and isolated lymphoid follicles.
Lymphoid organs
Anatomical site of lymphocyte development. In humans, these are the bone marrow (where B lymphocytes develop) and the thymus (where T lymphocytes develop from precursor cells that have migrated from the bone marrow). Also called central lymphoid tissue.
Primary lymphoid tissue
A primary lymphoid organ in the upper part of the middle of the chest, just behind the breastbone. It is the site of T-cell development.
Thymus
The lymph nodes, spleen, and mucosa-associated lymphoid tissues. These are the tissues in which immune responses are initiated. The highly organized tissues such as lymph nodes and spleen are also often known as secondary lymphoid organs. Also called peripheral lymphoid tissue.
Secondary lymphoid tissues
Thin-walled vessel that carries lymph. ( ___ ) are part of the lymphatic system that transports lymph from tissues to secondary lymphoid tissues (with the exception of the spleen) and from secondary lymphoid tissues to the thoracic duct.
Lymphatic vessel(s) (lymphatic)
Mixture of extracellular fluid and cells that is carried by the lymphatic system.
Lymph
The continual migration of naïve lymphocytes from blood to secondary lymphoid tissues to lymph and back to the blood. An exception to this pattern is traffic to the spleen: lymphocytes both enter and leave the spleen in the blood.
Lymphocyte recirculation
A type of secondary lymphoid tissue found at many sites in the body where lymphatic vessels converge. Antigens are deliverd by the lymph and presented to lymphocytes within the lymph node to initiate adaptive immune responses.
Lymph node
The lymph node nearest to a site of infection, to which extracellular fluid containing antigen and cells from the site is transported.
Draining lymph node
An aggregation of mainly B cells in secondary lymphoid tissues. Naïve B cells must pass through follicles for their survival; after B cells have been activated by antigen they enter the follicles, where they proliferate and undergo somatic hypermutation and isotype switching.
Lymphoid follicles
Vessel that brings lymph into a lymph node.
Afferent lymphatic vessels
Vessel by which lymph and lymphocytes leave a lymph node en route to the blood.
Efferent lymphatic vessel
Area in secondary lymphoid tissue that is a site of intense B-cell proliferation, selection, maturation, and cell death. ( ___ ) form aroun follucular dendritic cell networks when activated B cell migrate into lymphoid follicles. The cellular and morphological events that from the ( ___ ) and take place there are called the ( ___ ) reaction.
Germinal center(s)
An organ situated adjacent to the cardiac end of the stomach. One function of the ( ___ ) is to remove old or damaged red blood cells from the circulation; the other is as a secondary lymphoid organ in which immune responses to blood-borne pathogens and antigens are initiated.
Spleen
Bacteria that possess thick carbohydrate coats that protect them from phagocytosis. ( ___ ) cause extracellular infection and can be dealt with by phagocytes only if the bacteria are first coated with antibody and complement.
Encapsulated bacteria
Any inherited or acquired disorder in which some part or parts of the immune system are either absent or defective, resulting in failure to mount an effective immune response to pathogens.
Immunodeficiency disease
All lymphoid tissue closely associated with the gastrointestinal tract, including the palatine tonsils, Peyer's patches in the intestine, and layers of intraepithelial lymphocytes.
Gut-associated lymphoid tissues (GALT)
Large aggregates of lymphoid cells lying on each side of the pharynx.
Tonsils
Mucosa-associated secondary lymphoid tissues located in the nasal cavity.
Adenoids
Gut-associated secondary lymphoid tissue located at the beginning of the colon.
Appendix
Organized gut-associated lymphoid tissue present in the wall of the small intestine, especially the ileum.
Peyer's patches
The lymphoid cells and organized lymphoid tissues of the respiratory tract.
Bronchial-associated lymphoid tissue (BALT)
Aggregations of lymphoid cells in mucosal epithelia and in the lamina propria beneath. The main mucosa-associated lymphoid tissues are the gut-associated lymphoid tissue (GALT) and the bronchial-associated lymphoid tissue (BALT).
Mucosa-associated lymphoid tissue (MALT)
Specialized cell type in intestinal epithelium through which antigens and pathgoens enter gut-associated lymphoid tissue from the intestines.
Microfold cell (M cell)