immunology

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Immunology

IMMUNOLOGY

Sherko A Omer MB ChB, MSc., PhD

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Immunology

Recognition of foreign antigen is the character of adaptive immune response.

Two different molecules can recognize antigens: Immunoglobulins (Ig) T cell receptors (TCR)

MOLECULES THAT RECOGNIZE ANTIGEN

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Immunology

Antibodies (Immunoglobulin) are glycoproteins with antibody activity.

They combine specifically with the substances that may elicit them (immunogen or antigen).

Immunoglobulin form the humoral arm of immune response.

Immunoglobulin constitute 20% of total plasma proteins and are produced by plasma cells.

IMMUNOGLOBULIN

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Immunology

Immunoglobulins consist from 82-96% polypeptide and 4-18% carbohydrate (glycoproteins).

Immunoglobulins are bifunctional molecules that can bind specifically with an antigen and initiate a variety of secondary functions like complement fixation and attachment to other cells, and these secondary functions are independent of their specificity for antigens.

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

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Immunology

Basic unit (monomer), each Immunoglobulin molecule contains at least one basic unit and this basic unit consists from four polypeptide chains.

Heavy chain, each basic unit contains two identical heavy polypeptide chains each with approximately 400* amino acids.

Light chain, each basic unit contains two identical light polypeptide chains each with approximately 200 amino acids.

IMMUNOGLOBULIN

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Immunology

Each polypeptide chain of immunoglobulin have an amino terminal region (V) variable and a carboxyl terminal region (C) constant region, these terms describe the considerable variability in amino acid sequence in these regions.

The polypeptide amino acids of are linked non adjacently to form globular regions called domains, heavy chain domains includes VH, CH1,CH2 and CH3 while the domains of light chain are VL and CL. An additional domain CH4 is seen in IgM and IgE.

IMMUNOGLOBULIN

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Immunology

Paratope (antigen binding) consist from small numbers of amino acids in the V region of both heavy and light chains.

Hinge region, an area in the heavy chain between CH1-CH2, this area is more exposed to enzymes and it is more flexible for movement of Fab arms, IgM and IgE have no hinge regions.

IMMUNOGLOBULIN

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Immunology

Fab and Fc, digestion of IgG by the enzyme papain produce 2 Fab (Fragment antigen binding) and one Fc (Fragment crystallisable).

F(ab`)2, digestion of IgG by the enzyme pepsin produce one F(ab`)2 which consist from 2 Fab and hinge region and 1 P Fc` that consist of a small peptide (small fragment).

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

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Immunology

Disulfide bond, chemical disulfide bonds (S-S) are formed between cysteine residues, the bonds are essential for normal three dimensional structure of the immunoglobulin, disulfide bonds may be inter-chain (between H and H, H and L, or L and L chains) or intra-chains

IMMUNOGLOBULIN

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Immunology

Five classes of Immunoglobulins are present according to the structure and antigenic characteristic of the

constant C region of the heavy chain: IgG (G for Gamma )

IgA (A for Alpha ) IgM (M for Muta ) IgD (D for Delta )

IgE (E for Epsilon )IgG have four subclasses (IgG1, IgG2, IgG3 and IgG4)

IgA have two subclasses (IgA1 and IgA2).

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

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Immunology

Light chains are divided into two types on the basis of antigenic structures, these are Kappa () and Lambda (), subtypes are present for lambda chain.

S value, a value for sedimentation coefficient of immunoglobulin according to Svedberg’s technique, the bigger molecular weight of that immunoglobulin the bigger the S value.

Ig polymer, some Immunoglobulin consists from more than one basic unit such as like dimeric IgA and pentameric IgM.

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

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Immunology

J chain, a polypeptide chain present in polymeric Immunoglobulins such as pentameric IgM and dimeric IgA .

Secretary component (SC), a small polypeptide present in secretory IgA.

IMMUNOGLOBULIN

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Immunology

Immunoglobulin isotype, antigenic differences that characterize the class and subclasses of heavy chain and type and subtypes of light chain.

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

Immunoglobulin allotype, genetically determined antigenic differences in Immunoglobulins that varies in different members of the same species, these differences are located in C region so that a particular isotype may have several alternative allelic structure.

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Immunology

Immunoglobulin idiotype, the antigenic determinant that distinguish variable region of Immunoglobulin from other variable region of other immunoglobulins.

IMMUNOGLOBULIN

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Immunology

IMMUNOGLOBULIN

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Immunology

Glycosylation, carbohydrates are present in different amounts as simple or complex side chains of the C region of heavy chain, J or SC chain. Carbohydrates play role in secretion of Immunoglobulins by plasma cells.

Complementarity-determining regions (CDRs), most of the differences among antibodies fall within areas of the V regions called CDRs on both light and heavy chains, that constitute the antigen binding site of the antibody molecule.

IMMUNOGLOBULIN

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Immunology

Major immunoglobulin in human serum accounting for 80% of total serum Immunoglobulins

There are 4 subclasses IgG (IgG1, IgG2, IgG3 and IgG4) each with either two or two light chains.

IgG (IgG1, IgG3 and IgG4) are only Immunoglobulins that pass placenta so play an important role in protecting the developing fetus.

IgG

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Immunology

IgG except IgG4 can fix complement through the classic pathway (lgG3 more than IgG1 and IgG2) using their CH2.

IgG1 and IgG3 bind with high affinity to Fc receptors on phagocytic cells and thus mediate opsonisation.

IgG4 has an intermediate affinity for Fc receptors, and IgG2 has an extremely low affinity

IgG

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Immunology

IgM accounts for 5% –10% of the total serum immunoglobulin, with an average serum concentration of 1.5 mg/ml.

Monomeric IgM, with a molecular weight of 180 000 Da, is expressed as membrane-bound antibody on B cells (mIg).

IgM is secreted by plasma cells as a pentamer in which five monomer units are held together by disulfide bonds.

IgM

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Immunology

IgM is the first immunoglobulin class produced in a primary response to an antigen.

It is the first immunoglobulin to be synthesized by the neonate, its presence fetal blood indicates intra-uterine infection.

Due to its pentameric structure, serum IgM has a higher valency than the other isotypes in binding antigens with many repeating epitopes such as viral particles.

IgM is also more efficient than IgG at activating complement

IgM

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Immunology

Because of its large size, IgM does not diffuse well and therefore is found in very low concentrations in the intercellular tissue fluids.

The presence of the J chain allows IgM to bind to receptors on secretory cells, which transport it across epithelial linings to enter the external secretions that bathe the mucosal surfaces.

IgM plays an important accessory role as a secretory immunoglobulin.

IgM

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Immunology

Constitutes only 10%–15% of the total immunoglobulin in serum.

Predominant immunoglobulin class in external secretions such as breast milk, saliva, tears, and mucus of the bronchial, genitourinary, and digestive tracts.

In serum, IgA exists primarily as a monomer, but polymeric forms (dimers, trimers, and some tetramers) are sometimes seen, all containing a J chain.

IgA

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Immunology

Binding of secretory IgA to bacterial and viral surface antigens prevents attachment of the pathogens to the mucosal cells, thus inhibiting viral infection and bacterial colonization.

Complexes of secretory IgA and antigen are easily entrapped in mucus and then eliminated by the ciliated epithelial cells of the respiratory tract or by peristalsis of the gut.

IgA

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Immunology

Secretory IgA has been shown to provide an important line of defense against bacteria such as Salmonella, Vibrio cholerae, and Neisseria gonorrhoeae and viruses such as polio, influenza, and reovirus.

Breast milk contains secretory IgA and many other molecules that help protect the newborn against infection during the first month of life. Because the immune system of infants is not fully functional, breast-feeding plays an important role in maintaining the health of newborns.

IgA

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Immunology

The potent biological activity of IgE allowed it to be identified in serum despite its extremely low average serum concentration (0.3g/ml).

IgE antibodies mediate the immediate hypersensitivity reactions that are responsible for the symptoms of hay fever, asthma, hives, and anaphylactic shock.

Localized mast-cell degranulation induced by IgE also may release mediators that facilitate a build up of various cells necessary for anti-parasitic defense.

IgE

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Immunology

IgD, has a serum concentration of 30g/ml and constitutes about 0.2% of the total immunoglobulin in serum.

IgD, together with IgM, is the major membrane-bound immunoglobulin expressed by mature B cells, and its role in the physiology of B cells is under investigation.

No biological effector function has been identified for IgD

IgD

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Immunology

The genes for Immunoglobulin molecules are located on different chromosomes.

Heavy chain genes are on chromosome 14.

light chain gens are on chromosome 22.

light chain are located on chromosome 2.

GENETICS OF IMMUNOGLOBULINS

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Immunology

The enormous numbers of Immunoglobulins are formed by somatic recombination of different genes.

For heavy chain there are different variable (V), diversity (D), joining (J) and constant (C) genes.

For light chain there are different V, J and C genes.

Different recombination of these genes gives different Immunoglobulins.


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Immunology


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Immunology

MONOCLONAL ANTIBODIES

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MONOCLONAL ANTIBODIES

Uses:

Measurement of proteins and drugs in the serum.

Tissue and blood typing.

Identification of infectious agents.

Identification of CD that can be used for classification and follow-up of leukaemia and lymphoma.

Identification of tumour antigens.

Identification of autoantibodies in a variety of diseases.

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Immunology

B CELL RECEPTOR (BCR) The mIg have very short cytoplasmic tails which are too short to be able to associate with intracellular signalling molecules.

(BCR) is a transmembrane protein complex composed of mIg and disulfide-linked heterodimers called Ig-/Ig-.

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Immunology

B CELL RECEPTOR (BCR) Molecules of this heterodimer associate with an mIg molecule to form a BCR.

The Ig- chain has a long cytoplasmic tail containing 61 amino acids; the tail of the Ig- chain contains 48 amino acids.

The tails in both Ig-/Ig- are long enough to interact with intracellular signalling molecules.

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Immunology

BCR

Some of the many signal-transduction pathways activated by the BCR

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Immunology

T CELL RECEPTORS (TCR)

Cells that express TCRs have approximately 105 TCR molecules on their surface.

TCR are exist as either and -T cell receptors.

Each chain in a TCR has two domains containing an intrachain disulfide bond that spans 60–75 amino acids.

The amino-terminal domain in both chains exhibits marked sequence variation, but the sequences of the remainder of each chain are conserved.

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Immunology


TCR domains include one variable (V) and one constant (C) that are structurally homologous to the V and C domains of immunoglobulins.

The TCR variable domains have three hypervariable regions, which appear to be equivalent to the CDRs in immunoglobulin light and heavy chains.

The majority of T cells in the human express T-cell receptors.

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Immunology


The T-cell receptor is closely associated with the CD3, a complex of polypeptide chains involved in signal transduction forming the TCR-CD3 membrane complex.

CD3 is a complex of five invariant polypeptide chains that associate to form three dimers: a heterodimer of gamma

and epsilon chains (), a heterodimer of delta and epsilon chains (), and a homodimer of two zeta chains () or a

heterodimer of zeta and eta chains ().

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Immunology


Schematic diagram of the TCR-CD3 complex, which constitutes the T-cell antigen-binding receptor.

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Immunology

GENETICS OF (TCR)

TCR germ-line DNA is organized into multigene families corresponding to the , , and chains. Each family contains multiple gene segments.

The mechanisms that generate TCR diversity are generally similar to those that generate antibody diversity.

The chain, is encoded by V, J, and C gene segments.

The chain is encoded by V, D, J, and C gene segments.

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Immunology

GENETICS OF (TCR)

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Immunology

CYTOKINES

Low-molecular-weight proteins that are produced and secreted by a variety of cell types.

They play major roles in the induction and regulation of the cellular interactions involving cells of the immune, inflammatory and hematopoietic systems.

Cytokines bind to specific receptors on the membrane of target cells, triggering signal-transduction pathways that ultimately alter gene expression in the target cells.

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Immunology

CYTOKINES

The susceptibility of the target cell to a particular cytokine is determined by the presence of specific membrane receptors.

A particular cytokine may bind to receptors on the membrane of the same cell that secreted it, exerting autocrine action; it may bind to receptors on a target cell in close proximity to the producer cell, exerting paracrine action; in a few cases, it may bind to target cells in distant parts of the body, exerting endocrine action.

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Immunology

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CYTOKINES

Cytokines fall into one of the following families: hematopoietins, interferons, chemokines, and tumor necrosis factors.

Cytokines act by binding to cytokine receptors, most of which can be classified as immunoglobulin superfamily receptors, class I cytokine receptors (also known as the hematopoietin receptor family) , class II cytokine receptors (also known as the interferon receptor family), members of the TNF receptor family, and chemokine receptors.

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Immunology

CYTOKINES

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Immunology

CYTOKINES

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Immunology

CYTOKINES

immunology

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