Immunology Definitions


Adaptive Immune Response

The response of antigen-specific lymphocytes to antigen, including the development of immunological memory. Adaptive immune responses are distinct from innate and non-adaptive phases of immunity, which are not mediated by clonal selection of antigen specific lymphocytes. 

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Afferent Lymphatic Vessels

Vessels of the lymphatic system that drain extracellular fluid from the tissues and carry antigen, macrophages and dendritic cells from the site of infection to lymph nodes or other peripheral lymphoid organs.

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Agonist Selection

A process by which T cells are positively selected for in the thymus by their interaction with relatively high affinity ligands.

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Allergic Reaction

A specific response to an innoculous environmental antigen, or allergen, that is caused by preexisting antibody or primed T cells. Allergic reactions can be caused by various mechanisms, but the most common is the binding of an allergen to IgE bound to mast cells, which causes the release of histamine and other biologically active molecules from the cell that cause the symptoms of asthma, hay fever and other common allergies.

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Alloantibodies and Alloantigens and Allogeneic

Alloantibody: Antibodies produced against antigens from a genetically non-identical member of the same species.

Alloantigen: Antigens from another genetically non-identical member of the same species. 

Allogeneic: Two individuals that differ at genes in the MHC

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Alternative Pathway

Complement activation pathway that is triggered by the presence of a pathogen in the absence of specific antibodies, and is thus part of the innate immune system. It leads to the production of complement protein C3b and its binding to the surface of the pathogen, after which the pathway is the same as the classical lectin pathways of complement activation.

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A protein that binds specifically to a particular substrate- called its antigen. Each antibody molecule has a unique structure that enables it to bind specifically to its corresponding antigen, but all antibodies have the same overall structure and are known collectively as immunoglobulins. Antibodies are produced by differentiated B cells in response to infection or immunisation, and bind to and neutralise pathogens or prepare them for uptake and destruction by phagocytes.

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Antigen Presenting Cells (APCs)

Highly specialised cells that can process antigens and display their peptide fragments on the cells surface together with other, co-stimulatory, proteins required for activating naive T cells. The main cells are dendritic cells, macrophages and B cells.

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Autoantibody and Autoantigen and Autocrine

Autoantibody: An antibody specific for self antigens

Autoantigen: A self antigen to which the immune system makes a response

Autocrine: cytokine or other biologically active molecule acting on the cell that produces it. 

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The digestion and breakdown by a cell of its own organelles and proteins in lysosomes. It may be one route by which cytosolic proteins can be processed for presentation on MHC class II molecules.

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B-1 and B-2 cells

B-1: A class of atypical, self-renewing B cells ( aka CD5 B cells) found mainly in the peritoneal and pleural cavities in adults. They have a much less diverse antigen-receptor retertoire than conventional B cells.

B-2: A name sometimes given to conventional B lymphocytes with highly variable antigen receptors.

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B cell/ B lymphocyte

One of the two types of antigen-specifc lymphocytes responsible for the adative immune responses, the other being the T cells. The function of B cells is to produce antibodies. B cells are divided into two classes B-1 and B-2. B-2 are the conventional type which are produced in the bone marrow throughout life, emerging to populate the blood and lymphoid tissues.

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B-cell Receptor (BCR)

The cell-surface receptor on B cells for specific antigen. It is composed of transmembrane immunoglobulin molecule associated with the invarient Igα and Igβ chains. On activation by antigen, B cells differentiate into plasma cells by producing antibody molecules of the same antigen specificity as this receptor. 

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Bone Marrow

The tissue where all the cellular elements of the blood are intially generated from haemopoietic stem cells. The bone marrow is also the site of further B cell development in mammals and the source of stem clles that give rise to T cells on migration to the thymus. Thus, bone marrow transplantation can restore all cellular elements of the blood, including the cells required for adaptive immune responses.

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C1 Complex

Protein complex activated as the first step of the classical pathway of complement activation. It comprises one molecule of the protein C1q bound to two molecules each of the proteases C1r and C1s. C1q initiates the classical pathway of complement activation by binding to the pathogen surface or to bound antibody. This binding activates the associated C1r, which in turn cleaves and activates C1s. The acitve form of C1s then cleaves the next two components of the pathway, C4 and C2.

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Complement Proteins

C1q, C1r, C1s: complement proteins that form the C1 complex in the first step of the classical pathway of compliment activation

C2: complement proteins of the classical pathway of activation that is cleaved by C1 into C2b and C2a. C2a is an active protease that forms part of the classical C3 convertase C4bC2a.

C3: complement protein on which all complement activation pathways converge. C3 is cleaved to form C3b, the principle effector molecule of the complement system, which binds covalently to the surface on which it is generated. Once bound it acts as an oposonin to promote the destruction of pathogens by phagocytes and removal of immune complexes.

C4: Complement protein of the classical pathway of activation; cleaved by C4b, which forms part of the classical C3 convertase. 

C5: Complement protein that is cleaved to release the pro-inflammatory peptide C5a and a larger fragment, C5b, that initiates the formation of the membrane attack complex from the terminal components of compliment. 

C6,C7,C8, C9: complement proteins that form a complex with C5b to initaite the late events in complement activation that induces cell lysis. The complex inserts into the membrane and induces the polymerisation of complement protein C9 to form a pore known as the membrane attack complex

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C3 and C5 convertase

C3 Convertase: Enzyme complex that cleaves C3 to C3b and C3a on the surface of a pathogen. The C3 convertase of the classical and lectin pathways is formed from the membrane bound C4b complexed with the protease C2a. The alternative pathway C3 convertase is formed from membrane-bound C3b complexed with the protease Bb.

C5 convertase: Enzyme complex that cleaves C5 to C5a and C5b

C4b-binding protein (C4BP)- A complement-regulatory protein that inactivates the classical pathway C3 converase formed by host cells by displacing C2a from the C4bC2a complex. it binds C4b attached to host cells, but cannot bind C4b attached to pathogens

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Clusters of differentiation (CD)

Groups of monoclonal antibodies that identify the same cell-surface molecule. The cell-surface molecule is then designated CD followed by a number e.g. CD1.

CD1: Small family of MHC class I-like proteins that are not encoded by MHC and can present glycolipid antigens to CD4 T cells.

CD3 complex: The invarient proteins CD3γ, δ and ε and the dimeric ζ chains, which form the signalling complex of the T-cell receptor.

CD4: the co-receptor of T-cell receptors that recognise peptide antigens bound to MHC class II molecules. It binds to the lateral face of MHC molecules.

CD8: The co-receptor for T-cell receptors that recognise the peptide antigens bound to MHC class I molecules.

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Cell-mediated Immune response

An adaptive immune response in which antigen-specific effector T-cells have the main role. The immunity to infection conferred by such a response is cell-mediated immunity. A primary cell-mediated immune response is the T-cell response that occurs the first time a particular antigen is encountered.

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Central Lymphoid organs/ tissue

The sites of lymphocyte development; in humans, these are bone marrow and thymus. B lymphocytes develop in the bone marrow whereas T lymphocytes develop in the thymus from bone marrow derived progenitors.

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Central Tolerance and Chemokine

Central Tolerance: Immunological tolerance to self antigens that is established while lymphocytes are developing in central lymphoid organs

Chemokine: A small chemoattractant protein that stimulates the migration and activation of cells, especially phagocytic cells and lymphocytes. Chemokines have a central role in inflammatory respones. 

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Clonal Deletion and Clonal Selection Theory

Clonal Deletion: The elimination of immature lymphocytes when they bind to self antigens, which produces tolerance to self as required by the clonal selection theory of adaptive immunity. Main mechanism of central tolerance.

Clonal Selection Theory: The central paradigm of adaptive immunity. It states that adaptive immune respones derive from the individual antigen-specific lymphocytes that are self tolerant. These specific lymphocytes proliferate in response to antigen and differentiate into antigen-specific effector cells that eliminate the pathogen, and into memory cells that sustain immunity.

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Complement System

A set of plasma proteins that act together as a defence against pathogens in the extracellular space. The pathogen becomes coated with complement proteins that facilitate its removal by phagocytes and that can also kill certain pathogens directly. Can occur through either the classical or alternative pathways.

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Cytotoxic Granules and Cytotoxic T cells

Cytotoxic Granules : Membrane enclosed granules containing the cytotoxic proteins perforin, granzymes, and granulysin, which are the defining characteristic of effector CD8 cytotoxic T cells and NK cells.

Cytotoxic T cell: Type of T-cell that can kill other cells. most are MHC class I-restricted CD8 T cells, but CD4 T cells can sometimes do this. Important in the hosts response against intracellular pathogens that live or reproduce in the cytosol

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A site of an antigen recognised by an antibody or an antigen receptor. A T-cell epitope is a short peptide derived from a protein antigen. It binds to an MHC molecule and is recognised by a particular T-Cell. B-cell epitopes are antigenic determinants recognised by B-cells and are typically structural motifs on the surface of an antigen

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Fc Receptor

Type of cell-surface receptor on macrophages and other cells in the immune system that binds Fc portions of immunoglobulins. There are different Fc receptors for different isotypes.

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Granulocytes and GM-CSF

Granulocytes: White blood cells with a multilobed nucleus and cytoplasmic granules. They comprise neutrophils, eosinophils and basophils.

GM-CSF- granulocyte-macrophage colony-stimulating factor, A cytokine involved in the growth and differentiation of cells in the myeloid lineage, including dendritic cells, monocytes and tissue macrophages and granulocytes.

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Helper CD4 T-cells, Helper T-cells

Effector CD4 T-Cells that stimulate B cells to make antibody in reponse to antigenic challeng:

TH2, TH1 and the Tfh subsets can preform this function

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Generic name for cytokines produced by Leukocytes. 

IL-1β: produced by active macrophages that has many effects in the immune response e.g. activation of vascular endothelium, activation of lymphocytes and induction of fever

IL-2: produced by activated naive T-cells and effection for their further proliferation and differentiation- key part of the adaptive immune response.

IL-4 and 5: cytokine characteristic of CD4 Th2 effector T cells

IL-10: produced by reg T cells which tends to supress lymphocyte responses

IL-17: promotes inflammation

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Innate Immune Response

That part of a response to an infection that is due to the presence of, and immediation activation of, the body's innate and relatively none specific defence mechanisms. It is what is first encountered by the pathogen, before adaptive immunity is induced.

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Heterodimeric cell-surface proteins involved in cell-cell and cell-matrix interactions. They are important in adhesive interactions between lymphocytes and antigen-presenting cells and in lymphocyte leukocyte adherence to blood vessel walls and migration into tissues.

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Cytokines that are induced in response to an infection. IFN-α and IFN-β are antiviral in their properties.

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Acidified organelles that contain many degradative hydrolytic enzymes. Material taken up into endosomes by phagocytosis or receptor-mediated endocytosis is eventually delivered to lysosomes.

Lysozyme: antomicrobial enzyme that degrades bacterial cell wall

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Major Histocompatability Complex (MHC)

A cluster of genes on human chromosome 6 thay encodes a set of membrane glycoproteins called the MHC molecules. The MHC also encodes proteins involved in antigen processing and other aspects of the hosts defence. The genes for the MHC molecules are the most polymorphic in the human genome, having large numbers of alleles at various loci.

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Natural Killer Cells

Large granular, Non-T, non-B lymphocyte, which kills virus infected cells and some tumor cell.s They bare a wide variety of invarient activating and inhibitory receptors, but do not rearrange immunoglobulin or T-cell receptor genes. They are important in innate immunity to viruses and other itracellular pathogens.

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Intracellular proteins with a nucleotide-oligomerisation domain (NOD) and a leucine-rich repeat domain that bind components of bacterial cell walls and activate the NFkB pathway, initiating inflammatory responses.

NOD-like receptors: Large family of proteins containing nucleotide-oligomerisation domain (NOD) associated with various other domains, and whose general function is the detection of microbes and of cellular stress.

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Opsonisation and Pattern Recognition Receptors and

Opsonisation: The coating of the surface of a pathogen by antibody and/or complement that makes it more easily digested  by phagocytes.

Pattern Recognition Receptors: Receptors of the innate immune system that recognise common molecular patterns on pathogen surfaces

PAMPS: Pathogen associated molecular patterns: Molecules specifically associated with groups of pathogens that are recognised by cells of the innate immune system

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Peripheral Tolerance and Plasma Cell

Peripheral Tolerance: Tolerance aquired my mature lymphocytes in the peripheral tissues.

Plasma Cell: Terminally differentiated activated B lymphocyte. Plasma cells are the main antibody-secreting cells of the body. they are found in the medulla of the lymph nodes, in spenic red pulp, bone marrow and mucosal tissues

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Reactive Oxidation Species (ROS), Respiratory Burs

Reactive Oxidation Species (ROS): Superoxide ion (O2-) and hydrogen peroxide H2O2, produced by phagocytic cells such as neutrophils after the ingestion of microbes, which help kill injested microbes.

Respiratory Burst: An oxygen requiring metabolic change in neutrophils and macrophages that have taken up opsonised particles, such as complement by phagocytosis. it leads to the production of toxic metabolites that are involved in killing engulfed microbes.

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Transporters associated with antigen processing. ATP-binding cassette proteins that form a heterodimeric TAP-1:TAP-2 complex in the ER membrane, through which short peptides are transported from the cytosol into the lumen of the ER, where they associate with MHC class I molecules.

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T Helper Cells

Th1- CD4 T-cells, activate macrophages and help stimulate B Cells to produce antibody

Th2- stimulate B cells - often called CD4 T helper cells

Th3: mucosal immune response and are presented orally. profuce Transforming Growth Factor- Beta which promotes the differentiation of Treg cells.

Th17: producing IL-17- help recruit neutrophils to the site of infection.

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Toll Like Receptors

Innate receptors on macrophages, dendritic cells and some others that recognise pathogens and their products e,g, LPS. This stimulates the receptor-bearing cells to produce cytokines that initiate the immune response

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