This document discusses antigens and their classification. It defines antigens as substances that can induce an immune response. Antigens are classified as either exogenous (external) or endogenous (internal) antigens. Exogenous antigens enter the body from the external environment, while endogenous antigens are further divided into xeno-genic, allogenic, and autologous antigens based on their origin. The document also discusses the properties of immunogens and antigens, as well as factors that contribute to immunogenicity.

2.  Substances that are capable of inducing
.
 They can be recognized by the surface
antibody(B cells) or by the T cell
receptors when associated with MHC
molecule.
 They can with the
antibody or T- cell receptor.

3. CLASSIFICATION OF ANTIGENS
EXOGENOUS ANTIGENS
Exogenous antigens are those
antigens which enter into the host body from their
surroundings or external environments. These are
basically of pollutants, microorganisms, pollens, drugs
etc.
ENDOGENOUS ANTIGENS
They are again classified onto –
Xeno-genic or Heterogenic antigens
Allogenic or Idiotypic antigens
Autologous antigens.

4. - Foreign
items which are related with tissue
transplantation and serology.
- Foreign
items which enter when an individual
receives a blood transfusion or undergoes
transplantation operation.
- This
group of antigens is very rare and unnatural.
In normal condition, self-components are
non-immunogenic in nature, but in an
abnormal condition self-body components are
started to be considered as non-self or
antigenic component.

5. – Ability to induce a
humoral or cell mediated immune
response.
– Ability to combine
specifically with the final products of
humoral or cell mediated immune
response.
 All molecules possessing the
property of immunogenicity also
possess the property of antigenicity.
 Some molecules that possess the
property of antigenicity but lack
immunogenicity is known as

6. Contribution of the Immunogen
Foreignness
Molecular size
Chemical Composition
Physical Form
Degradability
Contribution of the biological system
Genetic factors
Age
Method of administration
Dose
Route
Adjuvants

7. 1. Foreignness
The immune system normally discriminates between self and
non-self such that only foreign molecules are immunogenic.
When an antigen is introduced into an organism,
2. Molecular Size
There is not absolute size above which a substance will be
immunogenic. However, in general
it is likely to be.
The best immunogens tend to have a molecular mass
approaching 100,000 Da .The poor immunogens generally
have a molecular size b/w 5000 – 10000 Da.

9. 3.Chemical Composition
In general,
. The addition of
aromatic amino acids like tyrosine or phenyalanine to
synthetic polymers can have a profound effect on
immunogenicity. All four levels of protein can contribute to
the antigenic determininant and hence affect the
immunogenicity.
4.Physical form
In general particulate antigens are more
immunogenic than soluble ones and denatured
antigens more immunogenic than the native form.

10. 5. Degradability
Antigens that are
. This is because
for most antigens the development of an immune
response requires that the antigen be
phagocytosed, processed and presented to helper T
cells by an antigen presenting cell (APC).

11. 1. Genetic Factors
 Some substances are immunogenic in one species but not in another.
 Similarly, some substances are immunogenic in one individual but
not in others.
 The species or individuals may lack or have altered genes that
for antigen on B cells and T cells or they may not
have the appropriate genes needed for the APC to present antigen to
the helper cell.

12. 2. Age
 Age can also influence immunogenicity.
 Usually the very young and the very old have a
diminished ability to mount and immune response to an
immunogen.

13. 1. Dose
 The dose of administration of an immunogen can
influence its immunogenicity.
 There is a dose of antigen above or below which the
immune response will not be optimal.
 An insufficient dose will not stimulate an immune
response either because it fails to activate enough
lymphocytes or because it induces a nonresponsive
state.
 An excessively high dose also can fail to induce a
response because it causes lymphocytes to enter a non
responsive state.

14. 2. Route
Intravenous : into a vein
Intradermal : into the skin
Subcutaneous : beneath the skin
Intramuscular : into a muscle
Intraperitoneal : into the peritoneal cavity
 The administration route determines which immune organs and cell
populations will be involved in the response.
 Antigen administrated intravenously is carried first to the spleen, whereas
antigen administrated subcutaneously moves first to local lymph nodes.

15. 3. Adjuvants
 Substances that are mixed
with an antigen and injected
with it, serve to enhance the
immunogenicity of the
antigen.
They are often used to boost
the immune response when
an antigen has low
immunogenicity or when
only small amounts of an
antigen are available, limiting
the immunizing dosage.

16.  Aluminum potassium sulfate acts to increase antigen
persistence.
 When an antigen is mixed with alum, the salt precipitates
the antigen. Injection of this alum precipitate results in a
slower release of antigen from the injection site, so that the
effective time of exposure to the antigen increases from a
few days without adjuvant to several weeks with the
adjuvant.
 The alum precipitate also increases the size of the antigen,
thus increasing the likelihood of phagocytosis.

17. Freund’s water-in-oil adjuvants also function to prolong antigen persistence.
FREUND’S INCOMPLETE ADJUVANT
Contains antigen in aqueous solution; mineral oil, and an emulsifing agent such as
mannide monooleate, which disperses the oil into small droplets surrounding the
antigen; the antigen is then released very slowly from the site of injection.
FREUNDS COMPLETE ADJUVANT
Contains heat killed Mycobacteria in the water-in-oil emulsion. In addition, a
muramyl dipeptide component of the myobacterial cell wall activates macrophages.
Activated macrophages are more phagocytic.

18. • Alum and both Freund’s adjuvants
stimulate a local, chronic inflammatory
response with an increase in phagocytic
cells as well as lymphocytes.
• This cellular inflammatory at the site of
the adjuvant injection often results in
formation of a dense, macrophage rich
mass of cells called a granuloma.
• Because the macrophages in a granuloma
are activated, this mechanism also
enhance TH – cell activation.
• Other adjuvants such as synthetic
polyribonucleotides and bacterial
lipopolysaccharides stimulate nonspecific
lymphocyte proliferation and thus
increase the likelihood of antigen-
induced clonal selection of lymphocytes.

19. EPITOPES
• An antigen is the part of an antigen that is
recognized by the immune system specifically by
antibodies, B cells or T cells.
• They are also known as epitopes or antigenic
determinants.
• B and T cell recognize different epitopes on the
same antigenic molecule.
• In the case of protein antigens, an epitope may
involve elements of the primary, secondary,
tertiary and even quartenary structure of the
protein.
• In the case of polysaccharide antigens, excessive
side chain branching via glycosidic bonds affects
the overall three dimensional conformation of
individual epitopes.

20. • T and B cell exhibit fundamental
difference in antigen recognition.
• B cells bind antigen that is free in
solution, the epitopes they recognize
tend to be highly accessible sites on
the exposed surface of the
immunogen.
• T cells only recognize processed
peptides associated with MHC
molecule on the surface of antigen
presenting cell and altered self cells.

21. MITOGENS
• Mitogens are agents capable of
inducing cell division in a high
percentage of T or B cells.
• A mitogen can activate many clones of
T or B cells irrespective of their antigen
specificity. Hence they are also known
as polyclonal activators.

22. • Several common mitogens are sugar binding
proteins called lecithins which bind specifically
to different glycoproteins on the surface of
various cells including lymphocytes.
• Binding of lectin molecules to membrane
glycoproteins often leads to agglutination, or
clustering of the cells which may trigger
cellular activation and proliferation.
• Some mitogens preferentially activate B cells,
some preferentially activate T cells and some
activate both populations.
• Three common lectins with mitogenic activity
are concanavalin A, phytohemagglutinin(PHA),
and pokeweed mitogen (PMW).

23. • The lipopolysaccharide component of the gram
negative bacterial cell wall functions as a B cell
mitogen.
• The mitogenic activity of LPS is due to its lipid
moiety, which is thought to interact with the plasma
membrane, resulting in a cellular activation signal.

24. Superantigens
 They are the most potent T-cell mitogens known.
 Superantigens bind to residues in the V(variable)
domain of the T-cell receptor and to residues in Class
II MHC molecule outside of the antigen-binding cleft.
 In this way a superantigen can cross-link a T cell to a
Class II MHC molecule even when the TCR does not
recognize the bound antigenic peptide, leading to
activation of the T cell.
 Thus a superantigen can activate all T cells
expressing the V domain to which that superantigen
binds.
 Common superantigens include the staphylococcal
enterotoxins(Ses) and toxic shock syndrome
toxin(TSS1), which is produced by the gram positive
bacterium Staphylococcal aureus.