ANTIGENS  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 response  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 immunogenceity.  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 molecule.

Chemically Antigens Can Be  Protein (pure, lipoprotein or glycoproteins)  CH (polysaccharides and lipopolysaccharides)  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 residues  Conformotional epitopes T cells recognise only amino acid sequences in a protein T cells recognize peptides asscoiated with MHC Antigenic determinants are usually amino acids long