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CPHS 2015

Acquired immune deficiency syndrome (AIDS)

Disease caused by infection with the human immunodeficiency virus (HIV). It involves a gradual destruction of the CD4 T-cell population and increasing susceptibility to infection.

Acute-phase proteins

Plasma proteins made by the liver whose synthesis is rapidly increased in response to infection. They include mannose-binding lectin (MBL), C-reactive protein (CRP), and fibrinogen.

Acute-phase response

A response of innate immunity that occurs soon after the start of an infection and involves the synthesis of acute-phase proteins by the liver and their secretion into the blood.

Adaptive immune response

The response of antigen-specific B and T lymphocytes to antigen, including the development of immunological memory.

Adaptive immunity

The state of resistance to infection that is produced by the adaptive immune response.


Substance used to enhance the adaptive immune response to any antigen. To exert its effect an adjuvant must be mixed with the antigen before injection or vaccination.

Afferent lymphatic vessels

Vessels that bring lymph draining from connective tissue into a lymph node en route to the blood.


A measure of the strength with which one molecule binds to another via a single binding site.

Affinity maturation

The increase in affinity of the antigen-binding sites of antibodies for the antigen that occurs during the course of an adaptive immune response.


The clumping together of particles, usually caused by antibody or some other multivalent binding molecule interacting with antigens on the surfaces of adjacent particles. Such particles are said to be agglutinated. When the particles are red blood cells, the phenomenon is called hemagglutination. When they are white blood cells it is called leukoagglutination.


An antigen that elicits hypersensitivity or allergic reactions. Allergens are usually innocuous proteins that do not inherently threaten the integrity of the body.


A state of hypersensitivity to a normally innocuous environmental antigen. It results from the interaction between the antigen and antibodies or T cells produced by earlier exposure to the same antigen.

Alternative pathway of complement activation

One of three pathways of complement activation. It is triggered by the presence of infection but does not involve antibody. See also classical pathway of complement activation; lectin pathway of complement activation.

Anaphylactic shock

IgE-mediated allergic reaction to systemically administered antigen that causes circulatory collapse and suffocation due to tracheal swelling. Also called systemic anaphylaxis.


A type of glycoprotein molecule, also called immunoglobulin (Ig), produced by B lymphocytes, that binds antigens, often with a high degree of specificity and high affinity. The basic structural unit of an antibody is composed of 2 identical heavy chains and 2 identical light chains. Amino-terminal variable regions of the heavy and light chains form the antigen binding sites, whereas the carboxy-terminal constant regions of the heavy chains functionally interact with other molecules in the immune system. In an individual, there are millions of different antibodies, each with a unique antigen-binding site. Secreted antibodies perform the various effector functions, including neutralizing antigens, activating complement, and promoting phagocytosis and destruction of microbes. Membrane-bound immunoglobulin can be found on naïve B lymphocytes.


Originally defined as any molecule that binds specifically to an antibody, the term now also refers to any molecule that can bind specifically to an antibody or T-cell receptor. Antigens that bind to antibodies include all classes of molecules. TCRs only bind peptide fragments of proteins complexed with MHC molecules. See also epitope.

Antigen-binding site

The site on an immunoglobulin or T-cell receptor molecule that binds specific antigen.

Antigen presentation

The display of antigen as peptide fragments bound to MHC molecules on the surface of cells. This is the form in which antigen is recognized by most T cells.

Antigen-presenting cells

Cells that express either MHC class I and/or MHC class II molecules and thus display complexes of MHC molecule and peptide antigen on their surfaces. See professional antigen presenting cell.

Antigen processing

The intracellular degradation of proteins into peptides that bind to MHC molecules for presentation to T cells.

Antigen receptor

For a B cell, the antigen receptor is its cell-surface immunoglobulin; for a T cell the antigen receptor is a rather similar molecule called the T-cell receptor. Each individual lymphocyte bears receptors of a single antigen specificity.

Antigenic drift

A process by which point mutations in influenza virus genes cause differences in the structure of viral surface antigens. This causes year-to-year antigenic differences in strains of influenza virus.

Antigenic shift

A process by which influenza viruses reassort their segmented genomes and change their surface antigens radically. New viruses arising by antigenic shift are the usual cause of influenza pandemics.

Autoimmune disease

Disease in which the pathology is caused by an immune response to normal components of healthy tissue.

Autoimmune response

An adaptive immune response directed at an antigenic component of the individual's own body.


Adaptive immunity specific for an antigenic component of the individual's own body.

B cells

Lymphocytes that are dedicated to making immunoglobulins and antibodies.

B-cell receptor

The antigen receptor on B cells. Each B cell is programmed to make a single type of immunoglobulin. The cell-surface form of this immunoglobulin serves as the B-cell receptor for specific antigen. Associated in the membrane with the immunoglobulin are the signal transduction molecules Igα and Igβ.


Diverse prokaryotic microorganisms that are responsible for many infectious diseases of humans and other animals.

Bone marrow

The tissue in the center of certain bones that is the major site of generation of all the cellular elements of blood (hematopoiesis).


Foreign protein to which small non-immunogenic antigens, or haptens, can be coupled to render the hapten immunogenic. In vivo, self proteins can also serve as carriers if they are suitably modified by the hapten; this is important in allergy to drugs.

CD molecules

Cell surface molecules expressed on various cell types in the immune system that are designated by the "cluster of differentiation" or CD nomenclature.


A cell-surface glycoprotein on some T cells that recognize antigens presented by MHC class II molecules. CD4 binds to MHC class II molecules on the antigen-presenting cell and acts as a co-receptor to augment the T cell's response to antigen.

CD4 T cells

The subset of T cells that express the CD4 co-receptor and recognize peptide antigens presented by MHC class II molecules. See helper CD4 T cells.


A cell-surface glycoprotein on some T cells that recognize antigens presented by MHC class I molecules. CD8 binds to MHC class I molecules on the antigen-presenting cell and acts as a co-receptor to augment the T-cell's response to antigen.

CD8 T cells

The subset of T cells that express the CD8 co-receptor and recognize peptide antigens presented by MHC class I molecules. See cytotoxic T cells.

Cell-mediated immunity (cellular immunity)

Any adaptive immune response in which antigen-specific effector T cells dominate. It is defined operationally as all adaptive immunity that cannot be transferred to a naive recipient with serum antibody. Cell-mediated immune responses include CD4+ T cell-mediated activation of macrophages that have phagocytosed microbes and CD8+ cytolytic T lymphocyte killing of infected cells.


Large group of small proteins involved in guiding white blood cells to sites where their functions are needed. They have a central role in inflammatory responses.


Movement of a cell directed by a chemical concentration gradient. The movement of lymphcytes, PMNs, monoctyes, and other leukocytes inoto various tissues is often directed by gradients of chemokines.

Classical pathway of complement activation

One of three pathways of complement activation. It is activated by antibody bound to antigen, and involves complement components C1, C4, and C2 in the generation of the C3 and C5 convertases. See also alternative pathway of complement activation; lectin pathway of complement activation.

Clonal deletion

The elimination of immature lymphocytes that bind to self antigens. Clonal deletion is the main mechanism that produces self-tolerance.

Clonal selection

The central principle of adaptive immunity. It is the mechanism 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.

Common lymphoid progenitor

Stem cell that gives rise to all lymphocytes and is derived from a pluripotent hematopoietic stem cell.


A set of plasma proteins that act in a cascade of reactions to attack extracellular forms of pathogens. As a result of complement activation, pathogens become coated with complement components, which can either kill the pathogen directly or cause its engulfment and destruction by phagocytes. There are 3 pathways of complement activation that differ in how they are initiated. The classical pathway is activated by antigen-antibody complexes, the alternative pathway by microbial surfaces, and the lectin pathway by plasma lectins that bind to microbes. Each complement pathway consists of a cascade of proteolytic enzymes that generate inflammatory mediators and opsonins, and leads to the formation of a lytic complex that inserts into cell membranes.

Complement activation

The initiation by pathogens of a series of reactions involving the complement components of plasma, leading to the death and elimination of the pathogen. See also alternative pathway of complement activation; classical pathway of complement activation; lectin pathway of complement activation.

Complement receptors (CR)

cell-surface proteins on various cell types that recognize and bind complement proteins bound to antigens (chiefly C3b) Complement receptors on phagocytes facilitate the phagocytic engulfment of pathogens coated with complement. Complement receptors include CR1, CR2, CR3, CR4, and the receptor for C1q.

Conformational epitopes

Epitopes on a protein antigen that are formed from several separate regions in the primary sequence of a protein brought together by protein folding. Antibodies that bind conformational epitopes bind only to native folded proteins. Also called discontinuous epitopes.

Constant domains (C domains)

The constituent domains of the constant regions of immunoglobulin and T-cell receptor polypeptides.

Constant region (C region)

The portion of immunoglobulin (Ig) or T cell receptor (TCR) polypeptide chains that does not vary in sequence among different clones of B and T cells and is not involved in Ag binding. Fro immunoglobulins there are several versions of constant regions, but within each isotype (class of antibody) the sequence will be the same.


Proteins made by cells that affect the behavior of other cells. Cytokines made by lymphocytes are often called lymphokines or interleukins (abbreviated IL). Cytokines bind to specific receptors on their target cells.

Cytotoxic CD8 T cells

The subset of T cells that express the CD8 coreceptor and recognize peptide antigen presented by MHC class I molecules.

Cytotoxic T cells

T cells that can kill other cells. Almost all cytotoxic T cells are CD8 T cells. Cytotoxic T cells are important in host defense against viruses and other cytosolic pathogens, because they recognize and kill the infected cells.


Proteins made by cytotoxic T cells that participate in the destruction of target cells. Perforins, granzymes or fragmentins, and granulysin are examples of cytotoxins.


Peptides produced in epithelia and phagocyte granules, which act as broad-spectrum antibiotics that kill a wide variety of bacteria and fungi

Delayed-type hypersensitivity (DTH)

A form of cell-mediated immunity elicited by antigen in the skin and mediated by CD4 TH1 cells. It is called delayed-type hypersensitivity because the reaction appears hours to days after antigen is injected.

Dendritic cells

Professional antigen-presenting cells with a branched, dendritic morphology. They are the most potent stimulators of T-cell responses. Also known as interdigitating reticular cells, they are derived from the bone marrow and are 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.


The movement of white blood cells from the blood across blood vessel walls into tissues.


The existence of a large number of lymphocytes with different antigenic specificities in any individual (i.e. the lymphocyte repertoire is large and diverse).

Draining lymph node

The lymph node to which extracellular fluid collected at a site of infection first travels.


delayed-type hypensensitivity


Abnormal accumulation of fluid in connective tissue, leading to swelling.

Effector cells

The cells that perform effector functions during an immune response, such s secreting cytokines (e.g. helper T cells), killing microbes (e.g., macrophages, neutrophils, and eosinophils), killing microbe-infected host cells (e.g., CTL's), or secreting antibodies (e.g., dirrentiated B cells). B cell efector cells would be memory cells or plasma cells; T cell effector cells would be T helper cells or sytotoxic T cells.

Effector mechanisms

The physiological and cellular processes used by the immune system to destroy pathogens and remove them from the body.

Efferent lymphatic vessel

Vessel in which lymph and lymphocytes leave a lymph node en route to the blood.

Encapsulated bacteria

Bacteria that possess thick carbohydrate coats that protect them from phagocytosis. Encapsulated bacteria cause extracellular infections and can be dealt with by phagocytes only if the bacteria are first coated with antibody and complement.

Endocytic vesicle

Membrane vesicle that is pinched off from the plasma membrane and takes extracellular material into cells.


The uptake of extracellular material into cells by endocytic vesicles that form by pinching off pieces of plasma membrane.


A component of the outer leaflet of gram negative bacteria, also called lipopolysaccharide, which is released from dying bacteria, which stimulates many innate immune responses including the secretion of cytokines and induction of microbicidal activities in macrophages and the expression of adhesion molecules for leukocytes on endothelium.


The cell layer that lines blood vessels.


White blood cell that is one of the three types of granulocyte. It contains granules that stain with eosin and whose contents are secreted when the cell is stimulated. Eosinophils contribute chiefly to defense against parasitic infections.


The specific portion of a macromolecule antigen to which an antibody binds. In the case of a protein antigen recognized by a T cell, an epitope is the peptide portion that binds to a major histocompatibility complex molecule for recognition by the T cell receptor. Also called an antigenic determinant.


Red blood cell.


The movement of cells and/or fluid from within blood vessels to the surrounding tissues.


A proteolytic fragment of IgG that consists of the two Fab arms held together by a disulfide bond. It is produced by digesting IgG with pepsin.

Fab fragment

A proteolytic fragment of IgG that consists of the light chain and the amino-terminal half of the heavy chain held together by an interchain disulfide bond. It is called Fab because it is the Fragment with antigen binding specificity. In the intact IgG molecule the parts corresponding to the Fab fragment are often called Fab or Fab arms.

Fc fragment

A fragment of an antibody, resulting from proteolytic cleavage, that consists of the carboxy-terminal halves of the two heavy chains disulfide-bonded to each other. It is called Fc because it was the Fragment that was most readily crystallized in early studies of IgG antibody structure. In an intact antibody the part corresponding to the Fc fragment is called Fc, Fc region, or Fc piece.

Fc receptors

Cell-surface receptors for the Fc portion of some immunoglobulin isotypes. They include the Fc (FcRI) and Fc (FcRI) receptors.


A rise of body temperature above the normal. It is caused by cytokines produced in response to infection.

Framework regions

Relatively invariant regions within the variable domains of immunoglobulins and T-cell receptors that provide a protein scaffold for the hypervariable regions.


Single-celled and multicellular eukaryotic organisms, including the yeasts and molds, that can cause a variety of diseases. Immunity to fungi involves both humoral and cell-mediated responses.

Gene segments

Multiple short DNA sequences in the immunoglobulin and T-cell receptor genes. These can be rearranged in many different combinations to produce the vast diversity of immunoglobulin or T-cell receptor polypeptide chains. See also D gene segments; I gene segments; V gene segments.

Germinal center

Area in secondary lymphoid tissue that is a site of intense B-cell proliferation, selection, maturation, and death. Germinal centers form around follicular dendritic cell networks when activated B cells migrate into lymphoid follicles.


White blood cells with irregularly shaped, multilobed nuclei and cytoplasmic granules. There are three types of granulocyte: neutrophils, eosinophils, and basophils. They are also known as polymorphonuclear leukocytes.

Gut-associated lymphoid tissues (GALT)

Lymphoid tissues closely associated with the gastrointestinal tract, including the palatine tonsils, Peyer's patches in the intestine, and layers of intraepithelial lymphocytes.

Heavy chain (H chain)

The larger of the two component polypeptides of an immunoglobulin molecule. Each heavy chain has a variable domain and constant domains. The different antibody isotypes, Including IgM, IgG, IgD, IgA, and IgE, are distinguished by structural differences in their heavy chain constant regions. The heavy chain constant regions also mediate the distinctive effector functions on the antibody molecule.

Helper CD4 T cells

CD4 T cells are sometimes generally known as helper cells because their function is to help other cell types to perform their functions. The term helper T cell sometimes refers to TH2 cells only, the cells that help B cells to produce antibody.


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.

Herd immunity

The phenomenon whereby those people in a population who have no protective immunity against a pathogen are largely protected from infection when the majority of the population is resistant to the pathogen.

Hinge region

The region of immunoglobulin heavy chains that can assume multiple comformations, thereby imparting a flexibility in the orientation of the 2 antigen-binding sites. Because of the hinge region the Ab can simultaneously bind 2 epitopes that are anywhere within reach of one another.


A vasoactive amine stored in mast cell granules. Histamine is released when antigen binds to IgE molecules on mast cells and causes dilation of local blood vessels and contraction of smooth muscle, producing some of the symptoms of immediate hypersensitivity reactions. Anti-histamines are drugs that counter histamine action.


Itchy red swellings in the skin caused by IgE-mediated reactions. Also called urticaria or nettle rash.


The acronym for Human Leukocyte Antigen. It is the genetic designation for the human MHC. Individual loci are designated by capital letters, as in HLA-A, and alleles are designated by numbers, as in HLA-A*0201.

HLA class I molecules

The name for the human version of the MHC class I molecules.

HLA class II molecules

The name for the human version of the MHC class II molecules.


The highly polymorphic human MHC class I genes.


The highly polymorphic human MHC class II molecules. Each class II molecule is made from α and β chains encoded by A and B genes respectively. For example, the HLA-DPα and HLA-DPβ chains are encoded by the HLA-DPA and HLA-DPB genes respectively. All the genes are in the MHC.

HLA type

The combination of HLA class I and class II allotypes that a person expresses.


The directed migration of subsets of circulating lymphocytes into particular tissue sites. Lymphocyte homing is regulated by the selective expression of adhesion molecules on the lymphocytes and the tissue-specific expression of endothelial ligand for these homing receptors. These migrations typically involve the movement of naive T cells into secondary lymphoid tissues, or of effector T cells to an effector site.

Human immunodeficiency virus (HIV)

The causative agent of the acquired immune deficiency syndrome (AIDS). HIV is a retrovirus of the lentivirus family that infects CD4 T cells, leading to their slow depletion, which eventually results in immunodeficiency.

Human leukocyte antigen (HLA) complex

The human major histocompatibility complex (MHC).

Humoral immunity

The type of adaptive immunity that is mediated by antibodies. Humoral immunity is the principal defense mechanism against extracellular microbes and their targets. Humoral immunity can be transferred to a non-immune recipient by serum.

HV regions

hypervariable regions


Hybrid cell lines that make monoclonal antibodies of defined specificity. They are formed by fusing a specific antibody-producing B lymphocyte with a myeloma cell that grows in tissue culture and does not make any immunoglobulin chains of its own.

Hypersensitivity reactions

Immune responses to innocuous antigens that lead to symptomatic reactions on reexposure. These can cause hypersensitivity diseases if they occur repetitively. This state of heightened reactivity to an antigen is called hypersensitivity. Hypersensitivity reactions are classified by mechanism: type I hypersensitivity reactions involve the triggering of mast cells by IgE antibodies; type II hypersensitivity reactions involve IgG antibodies against cell-surface or matrix antigens; type III hypersensitivity reactions involve antigen:antibody complexes; and type IV hypersensitivity reactions are mediated by effector T cells.

Hypervariable regions (HV regions)

Small regions of high aminoacid sequence diversity within the variable regions of immunoglobulin and T-cell receptors. They correspond to the complementarity-determining regions.


The class of immunoglobulin having α heavy chains, IgA antibodies in dimeric form are the antibodies present in mucosal secretions. IgA in monomeric form is present in the blood.


The class of immunoglobulin having δ heavy chains. It appears as surface immunoglobulin on mature naive B cells but its function is unknown.


The class of immunoglobulin having ɛ heavy chains. It is involved in allergic reactions.


The class of immunoglobulin having γ heavy chains. It is the most abundant class of immunoglobulin in plasma.


The class of immunoglobulin having μ heavy chains. It is the first immunoglobulin to appear on the surface of B cells and the first antibody secreted during an immune response. It is secreted in pentameric form.

Immature dendritic cells

Dendritic cells present in tissues, which take up antigen but do not express co-stimulatory molecules and cannot yet act as professional antigen-presenting cells to naive T cells.


Resistant to infection.

Immune complex

The protein complex formed from the binding of antibodies to soluble antigens. The size of the immune complexes formed depends on the relative concentrations of antigen and antibody. Large immune complexes are cleared by phagocytes bearing Fc and complement receptors. Small soluble immune complexes tend to be deposited on the walls of small blood vessels, where they can activate complement and cause damage.

Immune system

The tissues, cells, and molecules involved in host defense mechanisms, primarily against infectious agents.


The ability to resist infection.


The deliberate provocation of an adaptive immune response by introducing antigen into the body.

Immunodeficiency diseases

A group of inherited or acquired disorders in which some part or parts of host defense are either absent or defective.

Immunoglobulins (Ig)

The antigen-binding molecules of B cells.

Immunological memory

The capacity of the immune system to make quicker and stronger adaptive immune responses to successive encounters with an antigen. Immunological memory is specific for a particular antigen and is long-lived.


A 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. This is also known as an inflammatory response. The cells that invade tissues undergoing inflammatory responses are often called inflammatory cells or an inflammatory infiltrate.

Inflammatory mediators

A variety of substances released by various cell types that contribute to the production of inflammation at a site of infection or trauma.

Innate immunity

The host defense mechanisms that act from the start of an infection and do not adapt to a particular pathogen. Also called the innate immune response.


Cytokines that help cells to resist viral infection. Interferon-α (IFN-α) and interferon-β (IFN-β) are produced by leukocytes and fibroblasts respectively, as well as by other cells, whereas interferon-γ (IFN-γ) is a product of CD4 TH1 cells, CD8 T cells, and NK cells. IFN-γ acts principally to activate macrophages.

Interleukin (IL)

A generic term used for many of the cytokines produced by leukocytes. See also IL-1, IL-2, IL-3, etc.

Isotype switching

The process by which a B cell changes the class of immunoglobulin made while preserving the antigenic specificity of the immunoglobulin. Isotype switching involves somatic recombination that attaches a different heavy-chain constant-region gene to the variable-region exon.


Classes of immunoglobulin—IgM, IgG, IgD, IgA, and IgE—each of which has a distinct heavy-chain constant region encoded by a different constant-region gene. The heavy-chain constant region determines the effector properties of each antibody class.

Lectin pathway of complement activation

One of the three pathways of complement activation. It is activated by the binding of a mannose-binding lectin present in blood plasma to mannose-containing peptidoglycans on bacterial surfaces. See also alternative pathway of complement activation; classical pathway of complement activation.


A general term for a white blood cell. Lymphocytes, granulocytes, and monocytes are all leukocytes.

Light chain (L chain)

The smaller of the two types of polypeptide chain that make up immunoglobulins. It consists of one variable and one constant domain, and in the immunoglobulin molecule it is disulfide-bonded to a heavy chain. There are two classes of light chain, known as κ and λ.

Linear epitope

Epitope of a protein recognized by antibody that consists of a linear sequence of amino acids within the protein's primary structure.


Mixture of extracellular fluid and cells that is carried by the lymphatic system.

Lymph nodes

A type of secondary lymphoid tissue found at many sites in the body where lymphatic vessels converge. Antigens are delivered by the lymph and presented to lymphocytes within the lymph node where adaptive immune responses are initiated.

Lymphatic system

A system of vessels throughout the body that collects tissue fluid (lymph) originally derived from the blood, and returns it, via the thoracic duct to the circulation. Lymph nodes are interspersed along these vessels to trap and retain antigens present in the lymph.

Lymphatic vessels (lymphatics)

Thin-walled vessels that carry lymph from tissues to secondary lymphoid tissues (with the exception of the spleen) and from secondary lymphoid tissues to the thoracic duct.


A class of white blood cells that consist of small and large lymphocytes. The small lymphocytes bear variable cell-surface receptors for antigen and are responsible for adaptive immune responses. There are two main classes of small lymphocyte—B lymphocytes (B cells) and T lymphocytes (T cells). Large granular lymphocytes are natural killer (NK) cells, lymphocytes of innate immunity.


Containing lymphocytes, or pertaining to lymphocytes.

Lymphoid lineage

All types of lymphocyte, and the bone marrow cells that give rise to them.

Lymphoid organs (lymphoid tissues)

Organized tissues that contain very 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 responses are initiated, are the lymph nodes, spleen, and mucosa-associated lymphoid tissues such as tonsils, Peyer's patches, and the appendix.

Lymphoid progenitor

Stem cell in bone marrow cell that gives rise to all lymphocytes.


A membrane-bound, acidic organelle abundant in phaogocytic cells, which contains proteolytic enzymes that degrade proteins derived mainly form the extracellular environment.

Lytic granules

Intracellular storage granules of cytotoxic T cells and NK cells that contain perforin and granzymes.

M cells

Specialized cells in intestinal epithelium through which antigens and pathogens enter gut-associated lymphoid tissue from the intestines. Short for microfold cells.

Macrophage activation

Stimulation of macrophages, which increases their phagocytic, antigen-presenting, and bacterial killing functions. It occurs in the course of infection.


Large mononuclear phagocytic cells resident in most tissues. They are derived from blood monocytes and contribute to innate immunity and early nonadaptive phases of host defense. They function as professional antigen-presenting cells and as effector cells in humoral and cell-mediated immunity.

Major histocompatibility complex (MHC)

A cluster of genes on the short arm of human chromosome 6 that encodes a set of polymorphic membrane glycoproteins called the MHC molecules, which are involved in presenting peptide antigens to T cells.


mucosa-associated lymphoid tissues

Mannose-binding lectin (MBL)

An acute-phase protein in the blood that binds to mannose residues on pathogen surfaces and when bound activates the complement system. It is also known as mannose-binding protein (MBP) and mannan-binding lectin.

Mannose receptor

A carbohydrate binding receptor (lectin) expressed by macorphages that binds mannose and fucose residues on microbial cell walls and mediates phagocytosis of the organisms.

Mast cells

Large bone marrow derived cells found resident in connective tissues throughout the body. They contain large granules that store a variety of chemical mediators including histamine. Mast cells have high-affinity Fcɛ receptors (FcɛRI) that bind free IgE. Antigen binding to mast cell associated IgE triggers mast cell activation and degranulation, producing a local or systemic immediate hypersensitivity reaction. Mast cells have a crucial role in allergic reactions.

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