Antibody

CLS-415 Nada Mohamed Ahmed ,MSc, MT (ASCP)i

Structure and function

Antibody

Nada Mohamed Ahmed ,MD, MT (ASCP)i

Objectives

• Introduction • Definition to antibody • Structure of Ab • Sub class of Ab (types)• Function

Introduction• Soluble glycoproteins found in the γ globulin of serum

and sometimes in tissue fluids

• Ab specific recognition of Ag

• Immunoglobulin Ig determines biological structure of Ab .(poor Ab)

Ab determines the function of Igs

All Abs are must be Igs and not vice versa , some Igs are defective or poor Abs .

However , both terms are used interchangeably .

Antibodies

• Proteins that recognize and bind to a particular antigen with very high specificity.

• Made in response to exposure to the antigen.

• Each antibody has at least two identical sites that bind antigen: Antigen binding sites.

Definition

Definition of Antibodies

Belong to a group of serum proteins called immunoglobulins (Igs).

Valence of an antibody: Number of antigen binding sites. Most are bivalent.

Immunoglobulins

ANTIBODY STRUCTUREAn antibody molecule is composed of two identical Ig heavy chains (H) and two identical light chains (L), each with a variable region (V) & constant region (C).

Antibody Structure

Immunoglobulins are glycoproteins made up of: - Four polypeptide chains (IgG): a- Two light (L) polypeptide chains b- Two heavy (H) polypeptide chains - The four chains are linked by disulfide bonds

- Terminal portion of L-chain contains part of antigen binding site

- H-chains are distinct for each of the five Immunoglobulins

- Terminal portion of H-chain participate in antigen binding site

- The other (Carboxyl) terminal portion forms Fc fragment

Antibody Structure

Light chain : made of two domains

One variable domain VL determine specificity (Epitope)One constant domain CL they are two types of a.a. sequences in their constant

domain 1- kapa or 2- lamda but never combined

Heavey chain : made up 4 or 5 domains one variable domain VH 4 Or 3 constant domain determines diversity CH

Sub class of Ab (types)In the middle of heavy chain there is area “hing region”

flexible joint allow two Ag binding.

When a.a. sequence in heavy chain ( determine class and subclass of Ab)

μ = Μh = IgM γ=γh=IgG α= αIgAδ=δH=IgDε=εH=IgE

Antibodies or Immunoglobulins

* Five classes of Antibodies: IgG IgM IgA IgD IgE

General Antibody Functions

1. Opsonization of antigens.

2. Activation of the complement.

3. Neutralization of bacterial and viral toxins.

4. Enhancement or mediated allergic and

inflammatory reactions.

Immunoglobulin Classes IgGStructure: MonomerPercentage serum antibodies: 80%Location: Blood, lymph, intestineHalf-life in serum: 23 days

IgG function :Complement Fixation: YesCross Placenta :Yes

Enhances phagocytosis, neutralizes toxins and

viruses, protects fetus and newborn.

Immunoglobulin Classes. IgMStructure: PentamerPercentage serum antibodies: 5-10%Location: Blood, lymph, B cell surface (monomer)

Functions:Half-life in serum: 5 daysComplement Fixation: YesPlacental Transfer: No

Known Functions: 1-First antibodies produced during an infection.

2- Effective against microbes and agglutinating antigens.

Immunoglobulin Classes. IgAStructure: DimerPercentage serum antibodies: 10-15%Location: Secretions (tears, saliva, intestine, milk), blood and lymph.Half-life in serum: 6 daysComplement Fixation: NoPlacental Transfer: No

Known FunctionsPrevent attachment of bacteria and viruses to mucous membrane by

secretary peaces

Important for maternal immunity in colostrums during breast feeding

Immunoglobulin Classes. IgEStructure: MonomerPercentage serum antibodies: 0.002%Location: Bound to mast cells and basophils throughout

body. Blood.Half-life in serum: 2 daysComplement Fixation: NoPlacental Transfer: No

Known Functions 1-play important role in Allergic reactions and hypersensitivity . 2- lysis of worms.

Immunoglobulin ClassesIgDStructure: MonomerPercentage serum antibodies: 0.2%Location: B-cell surface, blood, and lymphHalf-life in serum: 3 daysComplement Fixation: NoPlacental Transfer: No

Known Functions: In serum function is unknown. On B cell surface, initiate immune response.

Serve as Ag receptor on B cell.