An antigen is a protein or polysaccharide that triggers a specific immune response in a living organism, leading to the production of antibodies or sensitized T cells. Antigens can be categorized into complete antigens, which induce an immune response, and incomplete antigens (haptens), which require binding to a carrier protein to become immunogenic. The document also discusses characteristics and determinants of antigenicity, including foreignness, size, chemical composition, and physical form.

1. Antigen and its types
Anup Muni Bajracharya

2. Antigen
• An antigen is a substance
• usually protein in nature & sometimes polysaccharide
• which when introduced into a living animal evokes
specific immune response
• either by producing specific antibody or specially
sensitised T cells or both. T
• The antigen react specifically with the corresponding
antibodies in some detectable or observable way.
• Auto-antigens or self antigens- Antigens which are
present on the body’s own cells.
• Foreign antigens or non-self antigens - Antigens from
outside or on the non-self cells.

3. Types of antigen:
• Antigens are of two types:
• Complete antigens and
• Incomplete antigens (haptens).

4. Complete antigen or immunogen:
• These are the substances that could stimulate
the immune response.
• These are high molecular wt. ( more than
10,000)
• Proteins but some are polysaccharides.
• Two important properties includes
• Immunogenecity
• Specificity

5. • Immunogenecity:
• capacity to induce the formation of
corresponding antibodies.
• B cell + antigen = Plasma B cells (secrets antibody) + memory B cells
• Specificity:
• react specifically with the antibodies produced.
• When B cells get activated it secretes
antibodies and the antigens will combine to
these antibodies.

6. Incomplete antigens or Hapten:
• low molecular weight ( less than 10,000)
• usually non-protein substances
• unable to induce an immune response by itself
but
• can became immunogenic only when covalently
linked to proteins called carrier protein in-vitro or
in -vivo.
• However, haptens can react specifically with its
corresponding antibodies.
• example of such hapten is drugs like penicillin.

7. Haptens
Haptens can be immunogenic if they bind with carrier proteins because once they bind
with carrier protein, it increases the molecular weight (becomes larger molecule) and
thus it can induce the immune response.
Haptens + Carrier protein = Immunogen
Examples of haptens :
a) bacterial: capsular polysaccharide
of pneumococci
b) lipids: forssman, cardiolipin
c) drug & chemicals agent causing
allergic contact dermatitis & drug
hypersensitivity.

8. Heterophil antigen (Cross reactive antigen) :
• Heterophil antigen are antigen that are found in
unrelated plants or animals
• but are either identical or so closely related
• Antibodies to one will cross react with the other
or certain antigens of similar nature present in
different tissues of more than one species.
• Antibodies to these closely related antigens
produced by one species cross react with
antigens of other species.

9. Heterophil antigen (Cross reactive antigen) :
• Eg:
• a) Forssman ag
• b) Cross reacting microbial antigen
• c) Human RBC + E. Coli

10. Forssman antigen:
• The best known of the heterophil antigen: the
forssman antigen originally described as an
antigen present in most guinea pig tissue,
• It is lipoprotein polysaccharide complex
(guinea pig tissue) which when injected on rabbit
stimulated production of haemolysin for sheep
red cells.
Antibody

11. Cross reacting microbial antigen:
• A heterophil antigen present is Rickettsiae
causing typhus fever are shared by certain
strains of Proteus ( ox19, ox2), this forms the
basis of Weil Felix reaction in which serum of
patient suffering from typhus fever
agglutinates with strains of Proteus.

12. Human RBC +E coli:
• Human RBC of blood group B shares antigen
with E coli constituting another example of
heterophil antigen characteristics of antigen.

13. Determinants of Antigenicty
• Foreignness
The immune system normally discriminates between self and non-
self such that only foreign molecules or nonself are immunogenic.
• In general, the antigenicity of a substance is related to the degree of
its foreignness.
• Antigen from other individual of the same species is less antigenic
than from other species and antigens from related species are less
antigenic than those from unrelated (distinct) species.
• Eg: The serum of one rabbit injected to another does not induce
antibody production but to other animal will, thus the more foreign
the molecules are the more is the antigenic nature.

14. • Size
In general, the larger the molecule the more immunogenic
it is likely to be.
• A macromolecule in terms of immunogenecity is a
molecule that has a molecular weight of 10,000 or more.
• Larger molecule like haemocyanin (6.75 million) and
• other products with molecular weight 14000-60000 are
(tetanus toxoid, egg albumin, thyroglobulin) are highly
antigenic &
• substances less than 10,000 dalton are not antigenic or
weakly antigenic.
• Penicillin and aspirin are not good antigen since their
molecular weight is less than 1000 dalton.

15. • Chemical Composition
• In general, the more complex the substance is chemically, the
more immunogenic it will be.
• The antigenic determinants are created by the primary
sequence of residues in the polymer and/or by the secondary,
tertiary or quaternary structure of the molecule.
• Generally protein are more antigenic than lipid and
carbohydrates.
• But not all proteins are antigenic, gelatin , histones though
protein are non antigenic.
• Bacterial exotoxins and egg albumin are powerful antigens.
• More complex carbohydrate is immunogenic, especially if
bound to proteins e.g. cell wall antigens of gram-negative
bacteria.

16. • Physical form
• In general particulate antigens are more immunogenic
than soluble ones and denatured antigens more
immunogenic than the native form.
• Degradability
Antigens that are easily phagocytosed are generally
more immunogenic.
• 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 .