This document defines antigens and discusses their properties. An antigen is any substance that can stimulate antibody production. Immunogenicity refers to the ability to induce an immune response, while antigenicity refers to the ability to bind antibodies or T cell receptors. Antigens can be complete or incomplete (haptens) and are classified based on their structure and ability to elicit an immune response. Factors like molecular size, composition, and physical form influence a substance's immunogenicity. The document also covers antigen epitopes, classes of antigens, superantigens, and provides examples of commonly used adjuvants.

1. Antigens
-Swathi K

2. Contents
 What is an antigen?
 Immunogenecity v/s Antigenecity
 Types of Antigens
 Factors that affect immunogenecity
 Classes of antigen
 Superantigens
 Other antigens
 References

3. What is an antigen?
 Antigen- is any substance which can stimulate the
production of the antibody, when entered into the
host.
 They substances that can be recognized by the
surface antibody (B cells) or by the TCR when
associated with MHC molecules.
 Immunogen- A substance which induces a
detectable immune response.

4. Immunogenicity VS Antigenicity:
Immunogenicity – ability to induce an antibody and/or
cell-mediated immune response
Antigenicity – ability to combine with the final
products of the response (antibodies and/or T cell
receptor)
NOTE: Most immunogenic molecules are also
antigenic.

6. Types of Antigens
Based on the ability to carry out immunogenicity and
immunological reactivity, antigens may be classified
into-
 Complete antigen- can induce antibody formation
and produce a specific and observable reaction with
the antibody so produced.
 Haptens - a small molecule that is antigenic but not
(by itself) immunogenic.
 Antibodies can be made to haptens only after the
hapten is covalently conjugated to a large protein
“carrier/adjuvant”

7. Adjuvant: a substance that, when mixed with an
antigen and injected with it, serves to enhance the
immune response to the antigen.
Commonly used adjuvants:
1) Alum
2) Incomplete Freund’s adjuvant
3) Complete Freund’s Adjuvant
4) Bacterial Lipopolysaccharides

9. Factors that influence immunogenicity:
 Foreignness – non-self (far apart evolutionary or
phylogenetically)
 Type of molecule (chemical nature) - protein >
polysaccharide > lipid > nucleic acid –
 Molecular Size - >10,000 Daltons are more
immunogenic
 Composition - heterogeneity increases
immunogenicity. - 4ry > 3ry > 2ry > 1ry structure
 Degradability - protein antigens must be degraded
(phagocytosis) in order to be presented to helper T
cells.
 Physical Form - Denatured > Native

12. Epitope or Antigenic Determinant - the region of an
antigen that binds to a T cell receptor or a B cell
receptor (antibody).
Since an epitope is the part of the antigen that binds
to the B cell or T cell receptor, it is the part that
determines the antigenicity of the antigen - thus the
term “antigenic determinant”.
T and B cells recognize different epitopes on an
antigen

13. There are two general classes of antigens
 T cell dependent antigens (external)
 T cell independent antigens (internal)

14. There are two general classes of antigens
 T cell dependent antigens : presented by Antigen
Presenting Cells (APC’s). These are macrophages,
B-cells, and some dendritic cells
 T cell independent antigens: typically peptides
derived from any protein; an infected cell displays
“not-self” proteins and is, thus, an “altered self
cell”

15. There are two general classes of antigens
 T cell dependent antigens : these antigens are
presented in MHC-II; they are seen by T-cells with a
TCR and an associated protein called CD4
 T cell independent antigens: these antigens are
presented by MHC-I; they are seen by T-cells
with a TCR and an associated protein called CD8

16. There are two classes of T-cells
 TH have CD4 which interacts with MHC-II; thus,
CD4+ T-cells are “MHC-II restricted.”
 TH cells are “helper cells” that send signals (via
cytokines and surface proteins) to other cells of
the immune system. The TH cells function as the
“brain” of the immune system.

17. There are two classes of T-cells
 TC have CD8 which interacts with MHC-I; thus, CD8+
T-cells are “MHC-I restricted.”
 TC cells become cytotoxic T lymphocytes (CTL’s)
which attack “altered self-cells (e. g., infected cells.)
“Altered self-cells” are also called “target cells.”
They are the targets for the CTL’s cytotoxicity.

19. Super-antigens
 Molecules that are potent T lymphocyte mitogens
and simultaneously bind to class II MHC molecules.
They are often associated with staphylococcal
products and are involved in enterotoxemias and
toxic shock syndrome in humans.

20.  Superantigens (SAgs) are secreted proteins
(exotoxins) that exhibit highly potent lymphocyte-
transforming (mitogenic) activity directed towards T
lymphocytes

23. References
 Ananthanarayana and Paniker’s, Textbook of
Microbiology, 9th edition.
 Kuby’s Immunology

25. Important Questions
 1) Define antigen and write the factors influencing its
immunogenecity.
 2) What is a hapten?
 3) what are adjuvants? Give examples.
 4) Define superantigens.
 5) What are the classes of antigen?