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Immunoglobulins

Jun Dou (窦骏)

Department of Pathogenic biology and Immunology

School of Medicine, Southeast University

May 8，2020

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Immunoglobulin (Ig) are the critical

ingredients at every stage of a humoral

acquired immune response. When expressed

on the surfaces of resting B lymphocytes,

they serve as receptors that can detect and

distinguish among the vast array of potential

antigens present in an environment.

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The Ig that secreted as a result then

function as antibodies, traveling through

the tissue fluids to seek out and bind to the

specific antigens that triggered their

production.

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Antibody structure

•Two heavy chains

•Two light chains

*H chain---50KD

* L chain---25KD

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Antibody structure?

V region (domain)Regions vary greatly in

amino acid sequence in

different Abs, involved

in Ag recognition.

C region (domain)relatively constant in

sequence, engaging

the effector functions.

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amino terminus

carboxyl terminus

glucoprotein

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tertiary structure

Complementarity

Determining Region

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Complementarity

Determining Region

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Antibody structure

hydrolysis

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Antigen Antibody

Interactions

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The Sheet

Anti-parallel βSheet

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Immunoglobulin Supergene Family.

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monomer

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Generation of Secretory Immunoglobulins

IgA

dimer

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Immunoglobulin Subtypes

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Functions of Antibody

◼ Neutralize toxins

◼ Bind to bacterial surfaces and aid in

opsonization

◼ Bind to bacterial surfaces and aid in

complement killing

◼ Inactivate viral particles

◼ Provide immunity to the unborn

◼ Provide immunity to the newborn

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Functions of Antibody

•Neutralization

•Opsonization

•Complement Activation

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Neutralization toxicant

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Immunoglobulin Isotypes

◼ Isotypes are the antigenic features of a class

of immunoglobulin H (heavy chain).

◼ Example for µ chain is isotypical different

from chain.

◼ The genes for 1, 2, 3, 4, µ, 1, 2, ,Ɛ,

chains are present in the human genome, and

are therefore isotypes.

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◼ IgM Big, First, Complement

◼ IgD Membrane receptor

◼ IgG Secondary response, prototype

◼ IgA Secretions

◼ IgE Allergy (Wheeze)

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Isotype

Allotype

Idiotype

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◼ Allotype

◼ Allotypes are additional features of immuno-

globulines that vary among individuals

within a species and are under genetic

control.

◼ Allotypes can be shared by some members of

a species.

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◼ Idiotype

◼ The term idiotype refers to the unique V-

region amino acid sequences of the

homogeneous immunoglobulin molecules

produced by a single B cell clone.

◼ Thus, there are as many idiotypes as there are

B cell clones (perhaps about 108 in an adult).

It is now known that anti-idiotype Ab

specificity recognize sequences in the

hypervariable regions of the target Ab that

induced it.

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An anti-idiotype immune response (Ab or T

cell mediated) expresses its own idiotype

which in turn can be recognized as foreign

and an anti-idiotype immune response made

against this idiotype.

Jerne described a Network Theory which

proposes that a series of idiotype-anti-idiotype

reactions are partially responsible for

regulation of the immune response.

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How does the immune system

make millions or billions of different antibody molecules?

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Mechanisms for Generation of Diversity

◼ Random assortment of light and heavy chains.

◼ Multiple V region gene segments.

◼ Somatic rearrangement.

◼ Faulty joining.

◼ Somatic mutation of rearranged genes.

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How to determine Ig gene structure?

Genetic structure (DNA) Ab (peptide)

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How are Ig genes rearranged?

**H chain genes are rearranged first, followed by L chain genes

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Somatic Rearrangement of Ig genes occurs during B cell differentiation

Ig M IgD

proteins

(Heavy chains)

VH >100 DH 1-12 JH 1-14

V, D, J Rearrangement. If 100 V segments and 12 D

segments and 14 J segments. 100x12x14 = 16800

different combinations are possible.

C C C 3 C 2 C 1 C 2 C 4 C 1 C 2

Transcription

Translation

mRNA

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TCR genes construct and rearrangment

Southern blot analysis

EcoRI digesting PCR products

TCR— and genes use a strategy of

recombination similar to that of Ig genes.

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Primary and Secondary Responses. Specific Adaptive or Acquired Immunity.

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Primary and Secondary Immune Response to Antigen

Antigen A and Antigen BAntigen A

Primary

response

to A

Primary

response to B

Secondary response to A

Time

[Antibody]

New Types

of Antibodies

IgG

IgA

IgE

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Antibody Use in Medicine

◼ Detecting assays

◼ Drugs, chemicals, hormones, pregnancy

tests, HIV tests, etc

◼ Enzyme linked immunosorbent assay

(ELISA)

◼ Blood typing

◼ Microbial identification

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◼ Passive Antibody Therapy.

◼ e.g. Gamma globulins

◼ Imaging

◼ Immunotoxins. Cancer therapy

◼ Neutralize inflammatory cytokines. e.g.

antibody to TNF

◼ Catalytic antibodies.

◼ Etc.

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ELISA Western

Blot

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Polyclonal antibody Monoclonal antibody (McAb)

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Monoclonal

Antibody

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Blood typing by agglutination

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Immunoprecipitation

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Immunodiffusion

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Double

Immunodiffusion

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Immunoflourescence

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FACSOr

Flow

Cytometry

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Humoral Immunity In Summary:

◼ Antibody functions by binding antigen and by

directing the activity of complement and

phagocytes

◼ Antibody is a protein termed immunoglobulin

◼ There are several different Immunuglobulin

classes or isotypes depending on the heavy chain

expressed.

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◼ Antibodies can exist in millions or billions

of different forms (antigen specificities)

created via a process termed generation of

diversity.

◼ Antibody production results from B cell

proliferation of differentiation.

◼ B cell proliferation and differentiation

usually requires helper T cells.

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◼ Extracurricular works I. Please answer the concepts below:

◼ 1. Ig and Antibody(Ab)

◼ 2. Ig Isotype

◼ 3. Ig Allotype

4. Ig idotype

◼ 5. Monoclonal Ab and Polyclonal Ab

• 6. Neutralization

◼ 7. Opsonization

◼ 8. CDRs and VH

◼ 9. Fab and Fc

◼ 10. Noncovalent forces

◼ 11. Ig subtypes and sIgA

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◼ II. Please answer the questions below:

◼ 1. Please demonstrate the basic structure of

immunoglobulin.

◼ 2. What are the functions of antibodies?

◼ 3. What are the differences of primary and

secondary response for antibody production?

◼ 4. How to understand the passive Ab therapy?

◼ (hint: COVID-19 therapy)