Antigen: Any substance reacting
with the products of any specific
immune response (Ig or T cells)
Immunogen: Any substance capable
to induce a specific immune
All antigens are NOT Immunogenic.
For immunogenicity:
foreign substance
Cemically more complex
- Larger molecules
are better immunogens (no limit but
usually > 6000 Da)
Particulate antigens are better
immunogens than soluble antigens
Denaturated antigenes are more
immunogenic than the native forms
Easly degradable and phagocytosed
antigens are more immonogenic
Host genetic factors influence the
The age of the hosts influence also
the immune response
The dose and route of antigen
administration also influence the
immune response
Antigens can bind to
B cell receptors
T cell receptors
MHC molecules
Haptens: Small molecules, which
are not immunogenic but they
induce an immune response when
they are attached to a carrier
Chemically Antigens Can Be
Protein (pure, lipoprotein or
 CH (polysaccharides and
 Nucleic acids
* Lipids are not immunogenic but
some phospholipids or glycolipids
may stimulate T cells
Antigenic determinant or
epitope: the portion of the antigen
which binds to the the products of a
specific immune response
T dependent antigens
They stimulate antibody production
with the aid of T cells.
- Proteins
T independent antigens
They can directly stimulate B cells
without the help of T cells
Pollysaccharides (repeated same
antigenic sites)
Type 1 T independent antigenes
may activate B cells polyclonally,
type 2 NOT
Adjuvants: Enhance the biological
effects o an antigen, nonspecifically.
Antigenic determinants recognized by
B cells
Linear epitopes: 4-8 amino acid or sugar
Conformotional epitopes
T cells recognise only amino acid sequences
in a protein
T cells recognize peptides asscoiated with
Antigenic determinants are usually 12-22
amino acids long