DEFINITION AND PROPERTIES OF ANTIGEN

Antigen (Ag) - any substance, which is recognized by the mature immune system of a given organism –antigenicity - specific reactivity with cells or molecules of the immune system –immunogenicity - capability to elicit an immune response –tolerogenicity - capability to induce immunological tolerance

Antigenic determinant (epitope) part of the antigenwhich are recognized by a defined immunoglobulin (B cell receptor or antibody) or by T cell receptor COMPLEX ANTIGENS CONSIST OF THE CARRIER AND MULTIPLE ANTIGENIC DETERMINANTS (EPITOPES) Carrier part of the antigen directly not involved in connection with antibody These terms can only be used to describe the interaction of particular antigenic determinant and single immunoglobulin or T cell receptor

B cell epitope T cell epitope recognized by B cells proteins polysaccharides lipids DNA steroids etc. (many artificial molecules) cell or matrix associated or soluble recognized by T cells proteins mainly (8-23 amino acids) requires processing by APC

T-INDEPENDENT ANTIGEN TI-1 T-INDEPENDENT ANTIGEN TI-2 B cell activation without the help of T cells Strong crosslinking of BCR by repetitive polysaccharide or protein epitopes. Cytokine help provided by T, NK cells etc.TI-2 usually do not act as polyclonal B cell activators. Complete absence of T-cell help. No interactions no cytokines. At high concentration B-cell mitogens. Simultaneous activation of BCR and other receptors on B cells (i.e. LPS binding protein /CD14/TLR4) induces the B cells to proliferate and differentiate B cell B CELL ACTIVATION

carrier + hapten Small chemical structures cannot induce B cell response on their own (e.g. drugs, reactive compounds) hapten (i.e. DNP:dinitrophenyl) - + HAPTENS hapten primed

carrier specifichapten specificcarrier + hapten specific carrier + hapten antibodies Antibody response generated against a hapten-carrier conjugate

Factors influencing immunogenicity I. Foreignness Size Genetics –Species –Individual Responders vs non-responders Age

Factors influencing immunogenicity II. Dose Route –Subcutaneous > intravenous,> oral > intranasal Adjuvant –substances that enhance an immune response to an antigen (alum, LPS, Freund’s adjuvant, TLR ligands) COMPLEX EFFECTS depo effect – slow antigen intake by antigen presenting cells activation of innate immunity

Physical status - corpuscle (cell, colloid) or soluble - denaturated or native Degradability - antigen presentation by APC Factors influencing immunogenicity II.

Superantigens conventional antigen monoclonal/oligoclonal T cell response 1: :10 5 superantigen polyclonal T cell response 1:4 - 1:10 Microbial proteins that bind to and activate all the T cells in an individual that express a particular set or family of TCR molecules 10 7 – 10 8 / / activated T cells

Immunoglobulins Definition: Glycoprotein molecules that are present on B cells (BCR) or produced by plasma cells (antibodies) in response to an immunogen Immune serum Antigen adsorbed serum α1α1 α2α2 β + - albumin globulins Mobility Amount of protein γ

Immunoglobulin Structure heavy and light chains disulfide bonds – inter-chain – intra-chain hinge region carbohydrate disulfide bond C H1 VLVL CLCL VHVH C H2 C H3

Immunoglobulin Fragments: Structure/Function Relationships antigen binding complement binding site placental transfer binding to Fc receptors

Immunoglobulin Structure variable and constant regions hinge region domains –V L & C L –V H & C H1 - C H3 (or C H4 ) oligosaccharides hinge region carbohydrate disulfide bond C H1 CLCL VHVH C H2 C H3

Ribbon structure of IgG

Immunoglobulin Fragments Structure/Function Relationships Fab – antigen binding – valence = 1 – specificty determined by V H and V L papain Fc Fab Fc –effector functions

Immunoglobulin Fragments: Structure/Function Relationships Fab – antigen binding Fc – effector functions F(ab’) 2 - Bivalent! pepsin Fc peptides F(ab’) 2

Why do antibodies need an Fc region? detect antigen precipitate antigen block the active sites of toxins or pathogen-associated molecules block interactions between host and pathogen-associated molecules the (Fab) 2 fragment can - inflammatory and effector functions associated with cells inflammatory and effector functions of complement the trafficking of antigens into the antigen processing pathways but can not activate (role of Fc region)

cell surface antigen receptor on B cells allows B cells to sense their antigenic environment connects extracellular space with intracellular signalling machinery secreted antibody neutralization opsonization complement fixation Immunoglobulin Structure-Function Relationship

(Classes/subclasses) Sequence variability of H/L- chain constant regions Sequence variability of H and L- chain variable regions (individual, clone- specific) Allelic variants Variability in different regions of the Ig determines Ig classes or specificity isotype idiotype allotype

Human Immunoglobulin Classes encoded by different structural gene segments (isotypes) IgG - gamma (γ) heavy chains IgM - mu (  μ ) heavy chains IgA - alpha ( α ) heavy chains IgD - delta ( δ ) heavy chains IgE - epsilon (ε) heavy chains light chain types kappa (κ) lambda (λ)

PRODUCTION OF IMMUNOGLOBULINS

secondary response against antigen A primary response against antigen A level of antibodies napok primary response against antigen B Antigen A days Antigen A and B

Polyclonal antibody response Ag Immunserum Polyclonal antibody Ag Set of B-cells Activated B-cells Antibody- producing plasma-cells Antigen-specific antibodies

Antibodies with different isotypes differ in their Binding affinity, effector functions and their Transport. Carbohydrate antigens are usually recognized By IgM type antibodies. Differences in transport makes all the differece: Antibodies spec. to blood group antigens

Structures of the ABO blood group antigens Defined by specific enzymes inherited co-dominant genes (Mendelian rules)

Donors and recipients for blood transfusion

Rhesus (Rh) blood group antigen (D) IgG type antibody- incomplete no direct agglutination but human immunglobulin-reactive 2. antibody can cause agglutination indirect agglutination POLYPEPTIDE TYPE ANTIGEN cytoplasm membrane extracellular space intracellular space

Pathological consequences of placental transport of IgG (hemolytic disease of the newborn)

Effects of agglutination in vivo ABO incompatibilityintravascular haemolysis ( complement mediated haemolysis) Rh incompatibilityhaemolytic disease of the newborn (erythroblastosis fetalis) ( opsonisation of red blood cells, which are then phagocytosed by macrophages and granulocytes) Rh profilaxis