3. Antigens or immunogens and Antibodies.pptx

Immunogens or Antigens
Immunogen or antigen:
* A foreign substance, when introduced into
human body, stimulate formation of specific
antibodies or sensitized lymphocytes
* Antigens have the ability to combine
specifically with antibodies produced or
sensitized T-lymphocytes induced

Haptens:
- Low molecular weight substances
- These substances not immunogenic by itself
- If couple to a larger carrier molecule (albumin, globulins), they
become immunogenic
- Examples :
simple chemicals and drugs:
penicillin, sulphonamid, aspirin, cosmetic, tranquillizers, neomycin
skin ointment

Epitopes or Antigenic determinants:
* Sites on or within antigen with which antibodies react
* Antibodies are specific for epitopes

Types of Antigens
Exogenous Antigens
1- Bacterial antigens:
a- Antigens related to bacterial cells
- Somatic antigen (O): part of cell wall gm –ve bacter.
- Capsular antigen: usually polysaccharide
- Flagellar Ag (H) : a protein made of flagellin
- Fimbrial Ag: surface antigens in fimbriated bacilli
b- Antigen secreted by bacteria:
- Exotoxins
- Enzymes
2- Viral antigens:
a- protein coat viral antigens
b- Soluble antigens (soluble nucleoproteins as in influenza)

Types Of Antigens
Endogenous antigens
Human tissue antigens:
a- Blood group antigens:
A, B and Rh antigens
b- Histocompatibility antigens:
Glycoprotein molecules on all nucleotide cells:
- Major histocompatibility complex antigens (MHC)
- Human leucocyte antigen (HLA)

Major Histocompatibility Complex Antigens (MHC)
* MHC has an important function in presentation of antigens to T-
cells
* Helper T-cells recognize foreign antigens on surface of APCs, only
when these antigens are presented in the groove of MHC II
molecule
* Cytotoxic T-cells will only recognize antigens, on the surfaces of
virus infected cells or tumor cells only when these antigens are
presented in the groove of Class I molecule (MHC restriction)

Superantigens (SAgs)
* They activate multiple clones of T-lymphocytes
* Bacterial toxins:
Staph. aureus toxic shock syndrome toxin (TSST) and enterotoxins
Strpt. pyogenes pyrogenic toxin A
* They have the ability to bind both class II MHC molecules and TCR
β chain
* They act as a clamp between the two, providing a signal for T-cell
activation

Superantigens (SAgs)
* They are active at very low concentration causing release of large
amounts of cytokines
* The massive T-cell activation and release of large amounts of
cytokines cause systemic toxicity
* This method of stimulation is not specific for the pathogen
* It does not lead to acquired immunity i.e no memory

Antigen Binding And Recognition Molecules
Antigens are recognized by and bind to:
1) B-cell receptors (BCR) :
- These are membrane-bound immunoglobulins
(IgM and IgD) on B-cells
- BCRs can be secreted in plasma as antibodies
2) T-cell receptors (TCR)
- α and β chains anchored to T-cells
- There is a groove which binds small peptides
presented by MHC on surface of APCs
3) MHC molecules
They are essential for presentation of peptides so that they can be
recognized and bind to TCRs

Factors influencing Immunogenicty
1-Foreigness :
Foreign substances are immunogenic
2- Molecular size:
High molecular weight increase immunogenicty
3- Chemical structure complexity:
High complexity increase immunogenicty
4- Route of administration:
Parenteral routes are more immunogenic to oral route

Factors influencing Immunogenicty
5- Method of administration:
a- Antigen dose:
Appropriate dose optimum antigenicty
Low dose low- zone tolerance
High dose high-zone tolerance
b- Adjuvant:
Substance when injected with an antigen
enhance immunogenicty

A. Antibodies:
• Formed by B-cells
• They are immunoglobulins which react specifically with the
antigen that stimulated their production
• Make up to about 20% of plasma proteins

A. Antibodies:
Structure and function
1. Light (L) and Heavy (H) polypeptide chains
 based on their molecular weights:
 two types: λ (Lambda) or к (Kappa) based on amino acid
differences in the constant region
 The amino part of each L-chain contains part of the
antigen binding site
 H-chains are distinct for each of the five), immunoglobuli
classes designated as µ (mu), ԑ (epsilonᵞ (gamma) ᵟ Delta)
 The amino terminal portion of the H-chain participates in
the antigen-binding site;
 The carboxyl group forms the Fc fragment with various
biologic activity

A. Antibodies:
The L – and H- Chains are subdivided into Variable
regions and Constant regions.
The regions are sub-divided into 3 dimensionally folded
repeating segments called domains.
L-Chain: Consists of – 1 variable domain & 1 constant
domain
H-Chain: Consists of – 1 variable domain & 3 or more
constant domains

A. Antibodies:
Variable regions:
Responsible for antigen binding
Have hyper-variable amino acid sequences that form the
antigen binding sites
Antigen binding involves vaan-dar-waals forces,
electrostatic forces, and other weak forces
Constant region:
responsible for biologic functions such as hypersensitivity
reactions, complement activation

A. Antibodies:
Immunoglobulin classes
A. IgG:
Each IgG molecule consists of two L chains and 2 H-
chains linked by disulphide bonds
Divalent
There are 4 classes based on amino acid sequence
differences in the Constant region
Predominant Ab in secondary immune responses
Important against bacterial and viral infections
Can cross the placenta
The most abundant in new borns

A. Antibodies:
A. IgM:
main Immunoglobulin produced in primary immune
responses
Present on the surface of all uncommitted B-cells
Has five H-L chain units ie has ten Fab units
Most efficient Ig in agglutination, complement fixation
and other Ab-Ag reactions
Important in bacterial and viral infections
Can be produced by a fetus undergoing infection
Cannot cross the Placenta

A. Antibodies:
A. IgA:
Main Ig in seretions ie saliva, tears, milk, etc
Protects mucous membranes from attacks
Bivalent
Exists in serum

A. Antibodies:
A. IgE:
Fc region binds to receptors on Mast cells,
Basophils, and Eosinophils
Important in allergic reactions
May appear in external secretions
Also important in helminth infection

A. Antibodies:
A. IgD:
Acts as antigen receptor when present on the
surface of certain B-lymphocytes
Present only in trace amounts in serum

3. Antigens or immunogens and Antibodies.pptx

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3. Antigens or immunogens and Antibodies.pptx