ANTIGENS Antigen Any substance reacting with the products

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 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 12 -22 amino acids long