The document defines key terms related to antigens and the immune system. It states that antigens are substances that can induce an immune response and are recognized by B cells, T cells, or antibodies. Antigens contain antigenic determinants or epitopes, which are the specific parts of an antigen recognized by the immune system. Epitopes can be conformational, involving the antigen's 3D structure, or linear, involving a continuous amino acid sequence. For an antigen to be immunogenic, it must be large in size, foreign to the host, and multivalent with multiple epitopes. Small molecules called haptens are not immunogenic on their own but can become so when attached to a carrier protein.

1. ANTIGEN
FEBBY PAYVA
ASSISTANT PROFESSOR
DEPARTMENT OF ZOOLOGY
ST. JOSEPH’S COLLEGE FOR WOMEN, ALAPPUZHA

2. ANTIGEN DEFINITION
Substances which can induce an immune response.
➔ The term antigen was originally used to indicate any
molecule that induced B cells to produce a specific
antibody (i.e. antibody generator).
➔ specifically recognised by the elements of adaptive
immunity, that is by B cells or T cells, or both.
➔ Ag are recognised not only by antibodies but also by
antigen specific T cell receptors.
◆ Immunoglobulins recognise intact antigen
◆ T cell surface receptors recognise processed antigen on the
surface of antigen presenting cells with MHC Class I or
Class II surface proteins.

3. Antigenic DETERMINANTS -EPITOPES
● An epitope is the part of an antigen that is
recognised by the immune system, specifically by
antibodies, B cells, or T cells.
● Each antigen will have several antigenic determinants
or epitopes
● The part of an antibody that recognises the epitope is
called a paratope.
● Antigenic determinant sites on antigens are highly
specific

4. Epitopes-2 types
● The epitopes of protein antigens are divided into two
categories
● conformational determinants and linear epitopes, based
on their structure and interaction with the paratope
● conformational epitope -composed of discontinuous
sections of the antigen amino acid sequence that
become placed side by side in the folded protein.
● interact with the paratope based on its 3-D surface
features and shape or tertiary structure.
● -----Most epitopes are conformational.

5. ➔ Linear determinants interact with the paratope based
on their primary structure.
➔ formed by a continuous sequence of amino acids from
the antigen.

6. ➔ Antigenic macromolecules typically contain multiple
determinants, each of which can be bound by an
antibody. The multiple determinants may be identical
repeats or with different specificity. The presence of
multiple identical determinants in an antigen is
referred to as polyvalency or multivalency

7. CHARACTERISTICS of antigen
❖ Large molecular size
❖ It may be proteins, carbohydrates, lipids and even
nucleic acids.
❖ High molecular weight materials are better antigens than
those of low molecular weight.
❖ A substance
➢ below a molecular weight of about 5000 Daltons is not
likely to be a good antigen.
➢ with a molecular weight less than 10,000 Daltons are only
weakly immunogenic or not immunogenic at all.
➢ The most potent immunogens are macromolecular proteins
with molecular weights of 10,000 Daltons and more.

8. ❖ All large molecules are multi determinant. ---may be
identical/ different
❖ 1st prerequisite for immunogen- genetically foreign to the
host.The more foreign a substance is to a particular
species the more likely it is to be a powerful antigen.
❖ A substance which acts as an antigen in one species may
not do so in another because it is a natural component of
the second species. For example, egg albumin is an
excellent antigen in rabbits, but it fails to induce an
antibody response in fowl.
❖ Sometimes body constituents are recognised as foreign
leading to autoimmune disease.

9. Complete Antigen
● Any antigen capable of stimulating the formation of
antibody with which it reacts in vivo or in vitro, is a
complete antigen. It is distinguished from incomplete
antigen (hapten). Complete antigens are able to induce:
● Immunogenicity - ability to stimulate the proliferation
of specific lymphocytes and antibody production
● Reactivity - ability to react with activated lymphocytes
and the antibodies released in response to them.

10. HAPTENS (Incomplete Antigens)
● Incomplete antigens (haptens) are small molecules -
peptides,nucleotides, and many hormones that are not
immunogenic on their own but become immunogenic when
attached to protein carriers.
● If a hapten links up with the body's protein, the
adaptive immune system may recognise the complex as
foreign and mount a harmful attack.
● These cannot activate B Cells on their own i.e. they are
not immunogenic.
● Thus, haptens have the property of antigenicity but not
immunogenicity.

11. ● To make a hapten immunogen -must be linked to a
macromolecule called a carrier. The hapten carrier
complex, unlike free hapten, can act as immunogen.
● Eg.example of a hapten - poison oak.
(When people are exposed to poison oak or related plants
for the first time, the hapten enters their skin,
oxidises, and bind with the proteins it finds. The immune
system recognises the hapten once it binds to a protein
and develops specific antibodies, which will react the
next time the body is exposed to poison oak. This immune
response causes the appearance of an itchy red rash which
can be quite uncomfortable.)

12. THANK YOU