DEEPA BABIN

Asst Prof,Microbiology

� Ab are glycoprotein molecule produced

by plasma cells in response to an Ag

and react specifically in an observable

manner

� FUNCTIONS-Ag binding

� Effector functions- Complement

fixtn,other cells fixation

› Soluble: secreted in blood and tissue

› Membrane-bound: found on surface of B-cell, also known as a B-cell receptor (BCR)

- BCR binds circulating antigen, activating the B-cell and forming plasma cells or memory B-cells

- Epitope-Ag

- Paratope-Ab

- Idiotype-Antigenic determinant on paratope

HV

Constant

Variable

heavyV

C

S-S Fab

Fab

Fc

� Monomer: A flexible Y-shaped molecule with four protein chains:

2 identical light chains

� 2 identical heavy chains

� Each heavy and light chain has a constant and variable region

� The variable region binds the antigen in a “lock-and-key” manner

� L chains : 2 forms – kappa (κ) & lambda (λ)

� Each molecule of Ig can have either κκκκ or λλλλ, but never both.

� Antibodies can also be divided into two regions based on their function

› Fab (fragment, antigen binding) region.

� Tip of the antibody

� Binds the antigen

› Fc (fragment, crystallizable) region

› Determines biological properties of Ig

molecule.

� Base of the antibody

� Can bind cell receptors, complement proteins and other molecules

H chain designated by Greek letter.

– 5 different types: IgA, IgD, IgE, IgG, IgM

› IgM µ (mu)

› IgD δ (delta)

› IgG γ (gamma)

› IgA α (alpha)

› IgE ε (epsilon)

� H chain also divided into VH & CH regions; the CH region is further divided into CH1, CH2 & CH3.

� Regions also called as DOMAINS :

- globular in shape

- stabilized by intrachain disulphide bonds

� Ag binding sites are located in the variable domains.

� IgG- PROTECTS BODY FLUIDS

� IgA- PROTECTS BODY SURFACE

� IgM- PROTECTS BLOOD STREAM

� IgE- MEDIATES REAGINIC HYPERSENSITIVITY

� IgD- Recognisation receptor for Ag

� Structure: Monomer� Percentage serum antibodies: 80%� Location: Blood, lymph, intestine� Half-life in serum: 23 days� Complement Fixation: Yes� Placental Transfer: Yes only Ab� Major Ab of secondary response, found both in

serum & body fluids.� 4 subclasses found in humans – IgG1, IgG2,

IgG3 & IgG4, each having a distinct type of gamma chain

� Functions: Enhances phagocytosis, neutralizes toxins and viruses, protects fetus and newborn.

� Structure: Pentamer� Percentage serum antibodies: 5-10%� Location: Blood, lymph, B cell surface (monomer)

� Half-life in serum: 5 days� Complement Fixation: Yes� Placental Transfer: No� primary immune response.� Functions: First antibodies produced during an infection. Effective against microbes and agglutinating antigens. Useful in the diagnosis of congenital infections like syphilis, rubella, HIV, dengue,toxoplasmosis etc.

� Structure: Dimer second most abundunt Ab

� Location: Secretions (colostrum,tears, saliva,

intestine, milk), blood and lymph.

� Half-life in serum: 6 days

� Complement Fixation: No

� Placental Transfer: No

� Occur in 2 forms : IgA1 & IgA2

� Secretory IgA is always in dimeric form – composed of 2 basic chain units, a J chain & the secretory

component.

� Secretory component helps to transport the dimer

from the submucosa to the mucosal cell surface

� Functions: Localized protection of mucosal

surfaces. Provides immunity to infant digestive

tract.

� Structure: Monomer resemble Ig G

� Percentage serum antibodies: 0.2%

� Location: B-cell surface, blood, and

lymph

� Half-life in serum: 3 days

�Complement Fixation: No

� Placental Transfer: No

� Functions: In serum function is unknown. Occurs along with Ig M on the surface of B

cell- initiate immune response.

� Structure: Monomer LOW LEVEL IN SERUM

� Percentage serum antibodies: 0.002%

� Location: linings of respiratory & intestinal tracts.Bound to mast cells and basophilsthroughout body. Blood.

� Half-life in serum: 2 days

�Complement Fixation: No

� Placental Transfer: No

� Functions: anaphylactic type of hypersensitivity ,Allergic reactions. Possibly lysis of worms.

› B cells develop from stem cells in the bone

marrow of adults (liver of fetuses).

› After maturation B cells migrate to lymphoid

organs (lymph node or spleen).

› Clonal Selection: When a B cell encounters

an antigen it recognizes, it is stimulated and

divides into many clones called plasma

cells, which actively secrete antibodies.

› Each B cell produces antibodies that will

recognize only one antigenic determinant.

Programmed cell death (“Falling away”).

› Human body makes 100 million lymphocytes

every day. If an equivalent number doesn’t

die, will develop leukemia.

› B cells that do not encounter stimulating

antigen will self-destruct and send signals to

phagocytes to dispose of their remains.

› Many virus infected cells will undergo

apoptosis, to help prevent spread of the

infection.

� Structurally similar proteins in serum seen in certain pathological conditions.

� Bence Jones protein in multiple myeloma – light chains of Igs.

� Cryoglobulinemia – formation of gel or ppton cooling the serum which redissolveson warming – in myelomas, SLE etc.

� An individual produces a large number of Abs to cope with the vast number of different Ags.

� This Ab diversity is due to the Ig genes.

� Genes coding for the variable & constant portions of the chains are separate

� One or only few genes code for C region whereas many genes code for the V region.

� Multiple V- region genes.

� V-J & V-D-J recombination.

� Junctional diversity1. Nucleotide addition – extra nucleotides may

get inserted between VH & D, and between D & JH segments

� Somatic mutation – point mutation in the genes for V domain.

�THANK U