Immunology: Antibody Structure and Function

What are Antibodies?

Specific proteins collectively referred to as immunoglobulins (glycoproteins)

Five major classes of antibodies;

IgG, IgM, IgA, IgD, IgE

Where are antibodies found?

Found in plasma, tears, saliva, colostrum, etc

How are antibodies produced?

By B cells in response to and against one specific antigen

Immunoglobulin structure

Four chain polypeptide: two heavy (H) chains and two light (L) chains (chains linked by disulfide bonds)

Variable region

Unique, N-terminal end, antigen binding, hypervariable loops (immunoglobulin fold)

Constant region

One or more, carboxy-terminal end

Fc Fragment

Carboxy-terminal portions of the two H chains, does not bind antigen, effector functions in immune response (opsonization, complement fixation)

Fab Fragment

One L chain & half of one H chain, binds antigen

Two different types of light chains:

Kappa (κ), lambda (λ)

Light chain characteristics

200-220 amino acids in length, constant portion and variable portion, Bence Jones proteins

5 different types of heavy chains:

IgG: γ (gamma), IgM: μ (mu), IgA: α (alpha), IgD: δ (delta), IgE: ε (epsilon)

Heavy chain characteristics

Variable region (first ≈ 110 amino acids), constant regions (remaining amino acids, three or greater regions with similar sequences)

Hinge region

Portion of H chain between CH1 & CH2 regions, flexibility (allows each Fab region to function independently

Carbohydrate portion

Increases the solubility of the immunoglobulin, protects against degradation, enhances function of Fc region

Isotype

Antibody classes (IgG, IgM, etc)

Allotypes

Genetically determined differences in antibodies between people

Idiotypes

Recognize different specific epitopes

IgM

∼5-10% of total Ig, pentamer, MW∼900,000 Da, monomer on surface of B cells, 6 day half life, μ heavy chains, 5 basic subunits, held together by J chain, 10 antigen binding sites

IgM function

Primary response antibody, first Ig class to appear in response to antigen stimulation, synthesized while antigen is present

First Ig class to be produced by maturing infant that can not cross placenta and most efficient of all Ig classes

IgM

IgG

∼70-75% of total Ig, MW ∼150,000 Da, 23 day half life, longest of all Ig, γ heavy chain, 2 binding sites

IgG subclasses

Differ in number and position of disulfide bridges between H chains

IgG Function

Can cross placenta, provide immunity for newborn, fix complement, opsonization, diffusion into extracellular spaces

IgA

∼10-15% of total Ig (in plasma), monomer MW ≈ 160,000 Da

IgA1

Predominantly in plasma, anti-inflammatory properties

Ig2

Predominantly in mucosal secretions, Dimer MW ≈ 385,000 Da

IgA Secretory Component

As IgA is transported through intestinal epithelial cells or hepatocytes it binds to a glycoprotein: secretory

component (protects IgA from digestion)

IgA Function

Predominant in tears, saliva, colostrum, breastmilk, intestinal fluids, immunity for breastfeeding infant, opsonin, does not activate complement

IgD

Low concentration in plasma (<1% total Ig), primarily found on cell membrane on unstimulated B cell surface, monomer MW ≈184,000 Da, 1-3 day half life, δ heavy chains

IgD Function

May play a role in B cell activation, maturation, and differentiation

Plasma IgD does not serve protective function

Does not fix complement, phagocytes do not bind to Fc portion, does not cross placenta

IgE

Trace plasma protein, least abundant Ig, MW ≈ 188,000 Da, ε heavy chains

IgE functions

Defense against parasites, allergies and anaphylaxis

Human immunoglobulin genes are found in three unlinked clusters:

Heavy chain genes on chromosome 14, κ chain genes on chromosome 2, λ chain genes on chromosome 22

Heavy chains variable-region genes:

V, D, and J

Heavy chains constant-region genes:

Set of C genes

Light chains variable-region genes:

V and J

Light chains constant-region genes:

Set of C genes

Through a random selection process, these individual segments are joined to commit that B cell to making antibody of a:

Single specificity

Small =

Soluble

Large =

Precipitating

Specificity of an antibody

Ability of an antibody to combine with its antigen instead of another antigen

Cross-reactivity of an antibody

If some determinants of the antigen are common to some other unrelated antigen the antibody may also react with that antigen

Antibody affinity

Affinity is the strength of a single interaction between an antibody's binding site and an antigen epitope

Antibody avidity

The cumulative, total strength of multiple simultaneous binding interactions between molecules, such as an antibody and an antigen

Goodness of fit

Antigen-antibody binding is based on the attractive forces between the antigen and antibody surfaces

Affinity=

Summation of attractive and repulsive forces

Types of Bonding

Hydrophobic bonds, hydrogen bonds, Van der Waals forces, electrostatic forces

What are Monoclonal Antibodies?

Purified antibodies cloned from a single B cell (clone