antibody structure

What are the four polypeptide chains of IgG?

Two identical heavy chains and two identical light chains

What bonds hold antibody chains together?

Disulfide bonds

What is the basic shape of an IgG antibody?

Y-shaped monomer

What are the two major regions of an antibody chain?

Variable and constant regions

Which region determines antigen specificity?

Variable region

Which region determines antibody class and effector function?

Heavy-chain constant region

What is an immunoglobulin domain?

A folded protein domain with a beta-sheet sandwich structure

What does Fab stand for?

Fragment antigen binding

What is the function of Fab?

Binds antigen

What does Fc stand for?

Fragment crystallizable

What is the function of Fc?

Interacts with Fc receptors and complement

What does the hinge region do?

Allows flexibility between Fab arms and Fc

What enzyme produces Fab and Fc fragments?

Papain

What fragments result from papain cleavage of IgG?

Two Fab fragments and one Fc fragment

What fragments result from pepsin cleavage of IgG?

One F(ab')2 fragment and degraded Fc

Which heavy chain defines IgM?

Mu

Which heavy chain defines IgG?

Gamma

Which heavy chain defines IgA?

Alpha

Which heavy chain defines IgE?

Epsilon

Which heavy chain defines IgD?

Delta

Which antibody is usually secreted first in a primary response?

IgM

What is the usual secreted structure of IgM?

Pentamer

Which antibody is most abundant in serum?

IgG

Which antibody crosses the placenta?

IgG

Which antibody is most associated with mucosal secretions?

IgA

What is the common secretory form of IgA?

Dimer

Which antibody is involved in allergies and parasite defense?

IgE

Which antibody is expressed with IgM on naïve mature B cells?

IgD

What is the J chain?

Protein that helps assemble polymeric IgM and IgA

What is an antibody isotype?

Antibody class defined by constant-region structure

What is an epitope?

The part of an antigen recognized by an antibody

What is a paratope?

The antibody site that binds an epitope

What does CDR stand for?

Complementarity-determining region

How many CDRs are in one antibody antigen-binding site?

Six: three from each variable domain

What do CDRs primarily determine?

Antigen-binding specificity

What is antibody affinity?

Strength of one antigen-binding interaction

What is antibody avidity?

Combined strength of multiple binding interactions

Why can IgM have high avidity?

Multiple antigen-binding sites allow multivalent binding

What is a conformational epitope?

An epitope formed by the antigen's three-dimensional shape

What is a linear epitope?

An epitope formed by a continuous amino acid sequence

What types of forces stabilize antibody-antigen binding?

Noncovalent forces

Name two noncovalent antibody-antigen interactions.

Hydrogen bonds and electrostatic interactions

What are steric constraints in antibody binding?

Physical shape or spacing that limits access to epitopes

What is unusual about camelid heavy-chain-only antibodies?

They lack conventional light chains

Can antibodies bind intact antigens without MHC presentation?

Yes, antibodies and BCRs bind native antigen directly