Immuno Week 4

what are the 2 forms of BCR

membrane bound

soluble

what is the structure of membrane bound BCR

transmembrane glycoproteins on B-cells

how many binding domains per molecule do BCRs have

2

how many binding domains per molecule do TCRs have

1

To signal the nucleus, each BCR heavy chain needs to be paired with what

Ig-α (=CD79a) and Ig-β (=CD79b)

what does Mature B-cell activation lead to

expansion

BCR will have the same Ag specificity as the parental B-cell

what are soluble BCRs

secreted antibodies

where are these antibodies secreted from

plasma cells and plasmablasts

what do B-cell clones secrete

antibodies of 1 isotype/class and 1 specificity

what decides isotype/class

AA sequence of heavy chain (Fc region)

what are the different isotypes/classes of BCRs

IgM, IgA, IgD, IgE and IgG

what is this

membrane bound BCR

what is this

soluble/secreted BCR

what determines if Ab is membrane bound or secreted

Alternative mRNA splicing

what do soluble Abs have

hydrophilic segment

what do Membrane-bound Abs have

hydrophobic transmembrane segment and AAA cytoplasmic tail

what is the structure of an antibody

Fab region

Fc region

Hinge region

Heavy-chain dimerisation through disulfide bonds

what does the Fab region contain

containing the variable region with antigen binding site

what does the Fc region contain

constant domains

what determines the antibody isotype (Ig: M, D, G, A, E)

the genes encoding the constant heavy chain region

what isotype do all B cells produce initially

IgM

then B-cells can switch from IgM to another class

what does different gene splicing lead to

different constant heavy chains

what is the function of IgG (subclasses: IgG1, IgG2, IgG3, IgG4)

Secreted during secondary response

Major form of circulating antibodies

what is the function of IgA (subclasses: IgA1, IgA2)

Major form of circulating antibodies in external secretions

what is the function of IgE

Triggers immediate allergic reactions

what is the function of IgM

Secreted during primary response

what is the function of IgD

Exact function unknown

does the fetus produce Abs

no

what is the exception to this

Late pregnancy B1 B-cells of fetus produce low affinity IgM

how does the fetus receive protection

Passive IgG from mother crosses placenta, providing some protection from infection

how else is maternal IgG transferred to the offspring

through colostrum & breast milk

what does IgM do

Activates classical complement pathway

Opsonisation of pathogens and phagocytosis

what is IgM associated with

immune response to antigenically complex blood-borne infections

10 binding sites per molecule (2 on each, pentamer)

what % of normal human serum Immunoglobulins does serum IgM make up

10%

what is the major site of IgM production

Plasmablasts in spleen

where is IgD found

Transmembrane Ag receptor Found on mature B-cells

what is the half life of IgD

Very-low concentration in plasma, half-life is 2-3 days

what does IgD do

Research suggests involvement in immune tolerance and mucosal defence against pathogens

what is the main Ig isotype in blood

IgG (3-4 week half life)

what does IgG do

Opsonizes pathogens

activates classical complement

induces phagocytosis

what are the 4 subclasses of IgG

IgG1, IgG2, IgG3, IgG4

how do IgG subclasses vary

based on the hinge region that joins the constant regions of the heavy chain

when is IgA produced

during a secondary response to Ag

where is IgA present

Predominates in mucosal surfaces

where can IgA opsonise Ags

in the respiratory, GI & reproductive tracts

what form do IgAs exist

in monomer form and dimer form

what is IgA bound to

poly Ig receptor on basolateral side of epithelial cells

when is IgA Transported to apical side

for secretion on mucosal surfaces

what is IgE usually bound to

Fc receptors of mast cells or basophils

what does IgE do

Provide immunity to parasites e.g. helminths, venom

Also evoke responses to allergens – mast cell degranulation

what do All naïve B-cells express

plasma membrane-bound IgM and IgD (= BCR)

when are Mature B-cells activated

upon encounter with a specific antigen

what happens after activation

they begin to proliferate and produce secretory IgM and IgD

how does isotype switching occur

Further activation by Ag or other stimuli (e.g. CD40L on T helper cells, cytokines) can rearrange genes encoding the constant region of the heavy chain

what do CD4 T-cells do

stimulate progeny of B-cells expressing IgM and IgD to produce Abs of different heavy chain isotypes/classes through CD40L-mediated signals and cytokines

does the new isotype bind to a different Ag

no, it will have the same binding specificity for the Ag

what happens in the T independent response

B cell directly activated by antigen

Production of IgM

No Helper T-cells needed

what happens in the T dependent response

Require T-cell contact to switch isotype

Antigens stimulate generation of long-lived plasma cells and memory B-cells

what is affinity maturation

During a B-cell response to an infection, Abs improve affinity to Ags

what does Activation induced cytidine deaminase (AID) do

induces point mutations which drives Somatic hypermutation

what happens to Affinity of antibodies specific for microbial proteins over time

it increases

what is the primary response

Response on first exposure to antigen

Smaller amount of antibody produced in primary immune response

what is the secondary response

Subsequent exposures to same pathogen

Increased heavy-chain isotype switching and affinity maturation

can all Abs activate complement

no

what will Ab subclasses interact with

only with their specific Fc receptor

what is an example of this

FCRI receptor expressed only on basophils and mast cells

hence IgE can only bind specifically to these cells and activate them

what happens when micrbones produce virulence factors that can bind specifically to some Ab isotypes

they can evade complement activation and opsonization through specific isotypes/subclasses

what are the 4 main antibody functions

Neutralizing antibodies

Activation/Inhibition of Leukocytes through Fc region of receptors

Opsonizing Pathogens

Complement (C’) Activation

how do neutralising antibodies work

Soluble antibodies can bind and block pathogenic proteins or toxins

what is the result of neutralising antibodies

pathogens then cannot bind or kill cells anymore

what is an example of neutralising Abs

Ab against spike protein of COVID – virus cannot bind to epithelial cells anymore

what are most neutralising Abs

IgG in the blood, on mucosal surfaces IgA

how does Activation/Inhibition of Leukocytes through Fc region of receptors work

Specific receptors on white blood cells can bind Fc part of antibodies

how have some bacteria have developed evasion strategies (virulence factors) to avoid the immune system

by producing their own Fc binding receptors that can bind and capture Ab

how does Opsonizing Pathogens work

Bacteria coated with antibodies to signal phagocytosis

Helps eliminate infected and tumour cells

what does the Ability of cross-linking of Fc receptor do

creates additional stimulation of phagocytosis

how does complement (C’) Activation work

Antibody binds pathogen and triggers classical C’

phagocytosis, target cell lysis

what is an epitope

Part of antigen, the peptide sequence that binds specifically to ABS (antigen binding site)

what is a paratope

Part of antibody, the sequence of the receptor that interacts with the Ag in the ABS

where are the 3 hypervariable regions (HV1, HV2 & HV3) located

within the variable region of both the heavy and light chain

what are HVs aka

Complementarity Determining Regions

e. HV1 = CDR1, …CDR2, …CDR3

what are HVs Critical for

providing antigen specificity for the BCR

what does CDR3 do

provides the most critical contact points between paratope and epitope

what are the 2 types of epitopes

linear (continuous) epitopes

conformational (discontinuous) epitopes

what is the structure of linear epitopes

All amino acids of the epitope binding to the paratope are next to each other

what is the structure of linear epitopes

Amino acids located in different parts of the amino acid chain come in proximity to form an antigen

why are AAs located in different parts in linear epitopes

depends on the folding of the native protein which may come from 2 or more stretches of aa sequence

what is affinity

Measures strength of interaction between epitope and antigen binding site

what is avidity

Measures overall strength of an antibody-antigen complex

what 3 major parameters is avidity dependent on

1. Binding affinity of Ab for the epitope

2. Valency of both the Ab and Ag

3. Structural arrangement of proteins in the complex

what is valence

The number of binding sites on an Ab or Ag

what valence is IgG

bivalent (valence = 2) because they are monomers with 2 paratopes

what valence is IgM

decavalent (valence = 10) because they are pentamers with 2 paratopes each