antigens & antibodies II

antigen binding site (FAB)

hypervariable region, complementary determining region (CDR), paratope

refer to sites within variable region that contact the antigen.

bifunctional molecules

antigen binding site in variable region confers specificity

biological activity in the constant region confers different roles for isotypes.

isotypes

IgA, IgD, IgE, IgG, and IgM with 5 different heavy chain constant regions named alpha (α), delta (δ), epsilon (ε), gamma (γ), or mu (μ) respectively.

light chain constant regions

designated as kappa (κ) or lambda (λ)

monomeric antibody

2 identical light chains covalently linked to 2 identical heavy chains, each with a variable and constant region.

subclasses

antibodies with small differences in amino acid sequences in heavy chain constant region, unique properties

(IgG1, IgG2, IgG3, IgG4 & IgA1, IgA1)

IgM, IgA, IgG, IgE, IgD

5 major antibody isotypes

IgA

alpha (α) symbol for heavy chain constant region

IgD

delta (δ) symbol for heavy chain constant region

IgE

epsilon (ε) symbol for heavy chain constant region

IgG

gamma (γ) symbol for heavy chain constant region

IgM

mu (μ) symbol for heavy chain constant region

light chain + heavy chain

the 2 chains of a monomeric antibody molecule

constant & variable

the two regions of the light & heavy chain

antigen binding site

located in the variable region of heavy & light chain

biological activity

located in the constant region of heavy & light chain

cross-reacting antibody

an antibody that can bind to an epitope that is similar to the original epitope - but binds with a lower affinity

• e.g. allergic to shrimp → allergic to cockroaches

pepsin

proteolytic cleavage of IgG leaves:

• 1 F(ab)₂ consisting of fragment with 2 antigen binding sites

• Fc fragment degraded

papain

proteolytic cleavage of IgG leaves:

• 2 Fab - 2 fragments of antigen binding sites

• 1 Fc fragment

biological function

the role of the Fc region

binds antigen (specificity)

the role of the Fab regions

secretion of antibodies

dendritic cells present antigen fragments/MHC to T cells → T cells are activated & secrete cytokines

B cells interact with antigen - need T cell interaction & cytokines to be activated & differentiate into plasma cells

plasma cells secrete antibodies

VDJ

3 components of the heavy chain variable region

VJ

2 components of the light chain variable region

hydrophobic, hydrophilic

biochemical interactions with antigens

antigen can come on & off

relevance of the biochemical interactions being hydrophobic and hydrophilic (not covalent)

3 names that refer to the component of the antibody that contacts the epitope

• hypervariable region

• complementary determining region (CDR)

• paratope

a secreted antibody doesn’t have a transmembrane region

specify the difference between a surface antibody on a B cell & an antibody secreted by a plasma cell

allotypes

variants inherited as different alleles

not all healthy members of a species inherit that particular —-

idiotype

sum of the idiotopes in that antibody

idiotope

epitope within the variable region of an antibody

anti-idiotypic

antibody that recognizes an idiotope

has been used with success in patients with some forms of B cell cancer

IgM

half-life: ~ 1 week (5 - 7 days)

% of serum antibodies: ~15

form on naïve B cell surface: membrane bound

isotype of isohemagglutinins

plasma cells

secretes pentameric IgM

regulates polymer formation

role of the J chain on IgM

complement activation

the major role IgM in an immune response to pathogens

RBCs, epithelial, endothelial

tissues that express antigens of the ABO system

anti-B

isohemagglutinins present in individuals who are Group A

anti-A

isohemagglutinins present in individuals who are Group B

anti-A, anti-B

isohemagglutinins present in individuals who are Group O

none

isohemagglutinins present in individuals who are Group AB

O type

universal donor

AB type

universal recipient

no isohemagglutinins

why is group AB designated as the universal recipient?

no A/B antigens

why is group O designated as the universal donor?

IgD

exist primarily as a membrane bound monomeric form

has same specificity as IgM

low detectable serum levels

naïve B cells

where is IgD primarily located?

~ 3 weeks

half-life of IgG1, IgG2, IgG4

1 week

half-life of IgG3

75

serum % of the total circulating IgG

polymorphisms allotypes (Gm)

term used to designate IgG allotypes

Brambell receptor (FcRB), neonatal Fc receptor (FcRn)

the receptor on the placenta that is required for the mother’s IgG to enter the fetal circulation - transplacental transfer of IgG

5 cells that express FcγR

• monocytes

• macrophages

• neutrophils

• B cells

• NK cells

IgG

major role in:

• opsonization by phagocytes

• antibody- dependent cell medicated cytotoxicity (ADCC)

• neutralization of viruses & toxins

• class complement activation (except 1 subclass)

perforin & granzymes

explain how interaction of Fcγ on a cell surface antigen with FcγR on NK cells leads to death of the cell expressing the Fcγ bound to the pathogen

antibody-dependent cell mediated cytotoxicity (ADCC)

mechanism where interaction of Fcγ on a cell surface antigen with FcγR on NK cells leads to death of the cell expressing the Fcγ bound to the pathogen

phagolysosome + activate NADPH oxidase + iNOS

explain how interaction of Fcγ on a cell surface antigen on a pathogen with FcγR on phagocytic cells leads to death of the cell expressing the Fcγ bound to the pathogen

opsonin-mediated phagocytosis

mechanism where interaction of Fcγ on a cell surface antigen on a pathogen with FcγR on phagocytic cells leads to death of the cell expressing the Fcγ bound to the pathogen

neutralizing antibody

antibody that binds to the pathogen sequence that the pathogen would use to enter & infect cells (blocks the binding of pathogens to host cells)

can activate classical complement if C1 binds 2 adjacent antibodies that are bound to pathogen

explain the role of IgG in the activation of classical complement pathway

IgG (whose variable region is bound to an antigen that is also bound by mIg antibody) binds to the low affinity FcγR, which has a cytosolic ITIM region = triggers a negative signal = B cell is not activated

specify the role of IgG in downregulating B cell activation when there is a high titer of IgG antibody in serum.

it will turn off the B cell too soon

why is it important that the antibody-antigen complex binds to a low affinity FcγR in the downregulation of B cells?

IgA

half-life: ~ 1 week

% concertation in serum: ~15%

locations: serum & mucosal

role: binds to & neutralizes pathogens

secretory IgA (sIgA)

IgA primarily exists as — in mucosa-associated lymphoid tissue

• found in mucosal areas- gastrointestinal tract & secretions tears, sweat, saliva), breast milk, and colostrum

IgA1, IgA2

IgA subclasses

Am

IgA allotypes

regulates polymer formation

the role of J chain in dimeric IgA

protects sIgA from degradation/cleavage

specify the role of the secretory component in sIgA

polymeric immunoglobulin receptor (pIgR)

a protein that mediates the transport of immunoglobulins across epithelial cells

• plays a crucial role in mucosal immunity by transporting IgA from the basolateral to the apical surface of epithelial cells

dimeric IgA to sIgA

1. B cells activated in lamina propria = plasma cells

2. plasma cells secrete dimeric IgA (D-IgA)

3. D-IgA binds to polymeric Ig receptor (pIgR)

4. complex pIgR-D-IgA endocytosed

5. complex is transported to lumen surface

6. @ the luminal side of epithelial cell, pIgR is cleaved

7. D-IgA retains a piece called secretory component

8. IgA is now called sIgA

9. secretory component protects sIgA from cleavage

IgE

monomeric

• involved in allergic reactions and defense against parasites

location: bound to FcεR on mast cells & basophils

3 cell types that express FcεR

• mast cells

• basophils

• eosinophils

antigen crosslinks 2 adjacent IgE antibodies bound to FceR

how to mast cells & basophils activated to release histamine?

crosslinking

2 IgE antibodies bind to one antigen @ the same time

6 allergic disorders in which IgE plays a role

• food allergies

• asthma

• systemic anaphylaxis

• hives

• allergic conjunctivitis

• rhinitis

IgE-helminth complexes

the complex that triggers the release of eosinophil armamentarium (major basic protein; eosinophil cationic protein)