IMMUNOGLOBULINSSTRUCTURE AND FUNCTION

Arpad Lanyi

BSc Public Health5th week, 2015

IMMUNOGLOBULINS

Definition

Glycoprotein molecules that are present on B cells (BCR) or produced by plasma cells (usually referred to as antibodies) in response to an immunogen (antigen that provokes immune response)

B CELL ACTIVATION

Immunoglobulin STRUCTURE

• 2x Heavy chain (light blue)

• 2x light chain (dark blue)

• Variable regions antigen binding

• Constant regions

hinge region

carbohydrate

disulfide bond

CH1

VL

CL

VH

CH2 CH3

FLEXIBILITY OF ANTIBODIES

AntibodyBCR (B cell receptor)

MEMBRANE BOUND!

Associated chains for signaling

Transmembrane domain

Cytoplasmic domain

Antigen recognition B cell activation

SOLUBLE (freely circulating)

Antigen binding effector functionsProduced by plasma cells

mIg sIg

s

s

s

s

s

s

s

s

s ss s

CH2

CH3s

s

s

s

s

s

s

s

ss

VL

VH

CL

CH1 ss

ss

s

s

s

s

ss

effektor funkciók

konstans domének

antigénkötés

variábilis domének

ANTIBODY DOMAINS AND THEIR FUNCTIONS

Ag

Constant domains

Effector functions

Antigen recognition

Variable domain

Ag

B cell

B CELL ACTIVATION

BCR oligomerization results in B cell activation, proliferation and differentiation

ANTIGEN BINDING

Antigen Binding Fragment (Fab)

Complement binding site

Placental transferConstant fragment (Fc)

Binding to Fc receptors on phagocytic cells

HYPERVARIABLE REGIONS

B cell development in the red bone marrow DNA recombination (somatic gene rearrangement) of gene segments encoding variable domains of heavy and light polypeptide chains is responsible for generation of B cells with highly variable specificity

Epitope

CDR1 CDR2CDR3

CDR1CDR2

CDR3

Light chain

Heavy chain

CDR = complementarity determining region = hypervariable region

The three-dimensional structure of immunoglobulin C and V domains

DIFFERENT VARIABLE REGIONS DIFFERENT ANTIGEN-BINDING SITES DIFFERENT

SPECIFICITIES

ISOTYPE (CLASS)

Sequence variability of H/L-chain constant regions

• 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

Each isotype has a distinct constant region and the isotype of the antibody determines the effector functions….

PHASES OF B CELL RESPONSE

YIsotype Switching during B Cell Development PE SWITCHING

Ig isotype Serum concentration

Characteristics, functions

12-14 mg/ml

Major isotype of secondary (memory) immune response

Complexed with antigen activates effector functions (Fc-receptor binding, complement activation

Trace

amounts

The first isotype in B-lymphocyte membrane

Function in serum is not known

Trace amounts

Major isotype in protection against parasites

Mediator of allergic reactions (binds to basophils and mast cells)

3-3,5 mg/ml

Major isotype of secretions (saliva, tear, milk)

Protection of mucosal surfaces

1-2 mg/ml

Major isotype of primary immune responses

Complexed with antigen activates complement

Agglutinates microbes The monomeric form is expressed in

B-lymphocyte membrane as antigen binding receptor

MAIN CHARACTERISTICS OF ANTIBODY ISOTYPES

IgG1-IgG4

IgA1-IgA2

Ig . C onc entra tion

na p o k

p rim er response

„A” a ntig né

IgM

IgGIgAIgE

Szekund er ’la syec ond a ry response

„A” és a ntig én

„B”

5 10 15 20 25 30

IgM

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

ANTIBODY PRODUCTION DURING THE PRIMARY AND THE SECONDARY IMMUNE RESPONSES

Antibody-mediated immune responses

EFFECTOR FUNCTIONS OF ANTIBODIES

• Fab part: NEUTRALIZATION

• Fc part:– OPSONIZATION followed by

• opsonized phagocytosis (macrophage; IgG)• ADCC (NK cell; IgG)• mast cell degranulation (parasite, allergy; IgE)

– COMPLEMENT ACTIVATION

NEUTRALIZATION

Antigen binding

Complement binding site

Placental transfer

Binding to Fc receptors

OPSONIZED PHAGOCYTOSISFlagging a pathogen

Antigen binding fragment (Fab) binds the pathogen the Fc region is accessible for Fc-

receptors of phagocytic cells, facilitating (speeding up) the

process of phagocytosis

Main opsonins:

• antibodies• Complement molecules• Acute-phase proteins (CRP, SAP)

Opsonization facilitate and accelerate the recognition of the pathogens by phagocytes

Phagocytes must express receptors for the opsonins:

IgG FcγRIC3b CR1

Antibody Dependent Cellular Cytotoxicity (ADCC)

(A) High-affinity FcRs on the surface of the cell bind antibodies before it binds to antigen. (mast cell)

(B) Low-affinity FcRs bind multiple Igs that have already bound to a multivalent antigen. (macrophage, NK cell)

MAST CELL DEGRANULATION

FcεRI+

IgE

The complement system

• The complement system is a set of plasma proteins that act in a cascade to attack and kill extracellular pathogens.

• Approximately 30 components: – activating molecules– complement receptors– regulator factors– membrane proteins wich inhibit the lysis of host cells

• Most of the complement proteins and glycoproteins are produced in the liver in an inactive form (zymogen). Activation is induced by proteolitic cleavage.

Amplification of the complement cascade

inactive precursors

limitedproteolysis

activating surfaceenzyme

Activating surface needed!

Pathways of complement activation

Cellular and Molecular Immunology, 7th ed., 2014 Elservier

Antigen binding

Complement binding site

Placental transferBinding to Fc

receptors

SUMMARY

Antigen binding

Complement binding site

Placental transfer

Binding to Fc receptors

FcRn on the placenta facilitate the transfer of maternal IgG to

the body of the fetus

IgG

IgM

IgA

A F T E R B IR T H

breas t milkIgA

0

1 0 0 %( a d u l t )

3 3y e a r

2 546 a d u l t9 1m o n t h

maternal IgG

B E F O R E B IR T H

PRODUCTION OF IMMUNOGLOBULINS

IgG transport is so efficient that at birth babies have as high a level of IgG in their plasma as their mothers

These transfers are a form of passive immunization. The babies protection by IgG and IgA is against those pathogen that the mother has mounted

The children are most vulnerable during the first year of life (esp.3-12m) when maternal IgGs have disappeared but the de novo synthesis is at low level

Maternal IgG is transported by the neonatal Fc receptor (FcRn) across the placenta to the fetus

The receptor FcRn transports IgG from the bloodstream into the extracellular spaces of tissues

IgG half-life• FcRn is also present in the adult and involved in protecting IgG from

degradation• Accounts for the long (3 week) half-life of IgG compared to other Ig

isotypes

• Therapeutic agents that are fused to IgG Fc regions take advantage of this property e.g. Enbrel (TNFR-Fc)

Pathological consequences of placental transport of IgG

(hemolytic disease of the newborn)

Passive anti-D IgG

anti-RhIgM

Transcytosis of dimeric IgA antibody across epithelia is mediated by the poly-Ig receptor (pIgR)