Immunoglobulins: Structure and Function

Definition: Glycoprotein molecules that are present on B-cells (BCR) or produced by plasma cells (antibodies) in response to an immunogen Immune serum Ag adsorbed serum α1α1 α2α2 β γ + - albumin globulins Electroforetic mobility Amount of serum protein Separation of serum proteins depending on their charge, size, molecular weight

Immunoglobulin Structure Heavy & 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 Ag Binding Complement Binding Site Placental Transfer Binding to Fc Receptors

Immunoglobulin Structure Variable & Constant Regions Hinge Region Domains –V L & C L –V H & C H1 - C H3 (or C H4 in IgM) Oligosaccharides Hinge Region Carbohydrate Disulfide bond C H1 CLCL VHVH C H2 C H3

Ribbon structure of IgG

mIg = BCR

HYPERVARIABLE REGIONS

Immunoglobulin Fragments: Structure/Function Relationships Fab –Ag binding –Valence = 1 –Specificity determined by V H and V L Papain Fc Fab Fc –Effector functions VHVH VLVL

Immunoglobulin Fragments: Structure/Function Relationships F(ab’) 2 –Antigen binding: valence=2, bivalence! 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 NEUTRALISATION

Cell surface antigen receptor on B cells (BCR) Allows B cells to sense their antigenic environment Connects extracellular space with intracellular signalling machinery Secreted antibody Neutralisation Arming/recruiting effector cells 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) responsible for antigen specificity Allelic variants Variability in different regions of the Ig determines Ig classes or specificity isotype idiotype allotype IgG – Gm allels Ig classes: IgG, IgA, IgE, IgD, IgM; subclasses: IgA1-2, IgG1-4 (light chain izotypes: κ, λ)

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 ) Izotypes!

Free pentameric IgM structure (”star-shape”) IgM binding to an antigen (”crab-shape”)

Different antibody isotypes

Valence: the number of bonds that a given antibody can form with one or more antigens Affinity: strength of a single bond between a given antigen and a given antibody Avidity: in case of an antibody, it means the combined strength of multiple bonds

BEFORE BIRTHAFTER BIRTH Breast milk IgA IgM IgG IgA ADULT YEARSMONTHS Maternal IgG IG ISOTYPE PRODUCTION OVER THE ONTOGENESIS 100% (ADULT) IgA in breast milk has an important bridge role!

Epithelial cell J C C S S S S C C S S S S C C ss Secretory IgA and transcytosis B J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss pIgR & IgA are internalised ‘Stalk’ of the pIgR is degraded to release IgA containing part of the pIgR - the secretory component J C C S S S S C C S S S S C C ss IgA and pIgR are transported to the apical surface in vesicles B cells located in the submucosa produce dimeric IgA Polymeric Ig receptors are expressed on the basolateral surface of epithelial cells to capture IgA produced in the mucosa

EFFECTOR FUNCTIONS OF ANTIBODIES 1)Neutralisation 2)Opsonization (facilitated phagocytosis) 3)ADCC 4)Complement activation ( see it later )

NEUTRALISATION

OPSONIZATION An opsonin is any molecule that enhances phagocytosis by marking an antigen for an immune response. In the picture opsonins are antibodies.

ANTIBODY DEPENDENT CELLULAR CYTOTOXICITY (ADCC)

Primary response against the B antigen Primary response Secondary response A” and „B” antigen „A” and „B” antigen „A” antigen days Immunoglobulin concentration ANTIBODY PRODUCTION DURING IMMUNE RESPONSE

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

Methods of immunisation Serum containing the specific antibody (usually IgG) Endagered subject The subject with specific antibody - Does not depend on the immune response of the recipient - Acts immediately - Short-term protection only (elimination of Ig’s!) II. Active immunisation Vaccination is a good example, when not antibodies but inactivated or attenuated pathogens or purified antigens from pathogens are administered sc. Immune response depends on the immune state of the recipient, immune protection needs time to develop, but long term protection is provided (memory cells). I. Passive

PASSZÍV IMMUNIZÁLÁS No activation of the immune system Acts immediately The protection is short-term only Elimination of Immunoglobulins Pooled intravenous immunoglobulin (IVIg) (Intratect, Intraglobin, Octagam, Gammagard) (approx. 59% IgG1, 36% IgG2, 3% IgG3, 2% IgG4 and maximally 5% IgA) PROTECTED SUBJECTS serum antibodies ENDANGERED SUBJECT Intravenous immunoglobulin

Intravenous immunoglobulin #1 Low dose: passive immunisation Indications: primary or secundary immune deficiency - congenital agammaglobulinaemia - severe combined immune deficiency (SCID) - Wiskott-Aldrich syndrome - multiplex myeloma or chronic lymphoid leukemia - premature babies - allogenic bone marrow transplantation - congenital HIV-infection (AIDS)

High dose: immune suppression The „physiologic” immunsuppressive agent! Especially useful in the autoimmune diseases of children, the only limit is the price. Indications: - immune thrombocytopenia (ITP) - dermatomyositis/polymyositis - myasthenic crisis (myasthenia gravis) - Guillain-Barré syndrome - graft versus host reaction (after transplantation) Intravenous immunoglobulin #2