Immunogenes or Antigens

Immunogens Or Antigens

Francis Ian L. Salaver, RMT

Immunogen

* A foreign substance, when introduced into human body, stimulate formation of specific

antibodies or sensitized lymphocytes Antigen

* Antigens have the ability to combine specifically with antibodies produced or sensitized T-lymphocytes induced

• All immunogens are antigens but not all antigens are immunogens.

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, sulphonamide, aspirin, cosmetic, tranquillizers, neomycin skin ointment

Epitopes or Antigenic determinants: • Sites on or within antigen with which

antibodies react

• A single antigen may have multiple epitopes

* Antibodies are specific for epitopes

1. Foreigness 2. Degradability3. Molecular Weight4. Structural Stability5. Complexity

the degree to which antigenic determinants are recognized as nonself by an individual’s immune system

A.1 Autologous antigens - found within the same individual; one’s own antigen; would stimulate the production of autoantibodies

A.2 Syngeneic antigen- found between genetically identical individuals; twins

A.3 Allogeneic/Homologous- found between individuals of the same species

the degree to which antigenic determinants are recognized as nonself by an individual’s immune system

A.4 Xenogeneic/Heterologous- found between different individuals of different species

A.5 Sequestered antigens- antigens that are not exposed to antibody-producing cells

A.6 Heterogeneic- occur to unrelated animals and plants species. Good example is the cardiolipid from the beef heart muscle and T. pallidum

the degree of being susceptible to breakdown or decomposition

E.g. vaccination (adequate dose of vaccine at appropriate intervals must be administered )

The higher the molecular weight, the better the molecule will function as antigen

The number of antigenic determinants is directly related to its size.

Molecular weight of 10,000 or higher. Hapten: Small foreign molecule that is not

antigenic. Must be coupled to a carrier molecule to be antigenic. Once antibodies are formed they will recognize hapten.

If a molecule is an effective antigen, structural stability is mandatory

the more complex an antigen, the greater is its effectiveness

Complex CHONs are better Ag than large, repeating polymers such as lipids, CHO, and nucleic acids, which are poor Ag

* 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

* 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

Complete Antigen Incomplete Antigen/ Hapten

TD-Ag (thymus dependent Ag )

TI-Ag (thymus independent Ag)

TD-Ag can stimulate B cell to produce Ab

with the help of T cell

Most of TD-Ag are protein Have many kinds of determinants Stimulate B cell to produce :IgG, IgM, IgA Have immune memory

TI-Ag can stimulate B cells to produce Ab without the help of T cell

Most are polysaccharide Only induce B cell to produce IgM Can not induce CMI No immune memory

1. Unideterminant, Univalent antigens- there is only one epitope present

2. Unideterminant, Multivalent- there is only one type of epitope present but many such epitopes on each molecule

3. Multideterminant, Univalent- there are many types of epitopes but only one of each kind per molecule

4. Multideterminant, Multivalent- there are many different kinds of epitopes and many of each kind per molecule