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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