ANTIGENS
Antigen (Ag) is any substance (chemical compounds of various nature such as peptides, hydrocarbons, polyphosphates, steroids) that potentially is capable of being recognized by the immune system as foreign ("non self") and promotes development of immunological reactions. The term "immunogen" is often used as a synonym for antigen. The capacity to cause an immunological response is not inherent for the entire Ag molecule, but only for its specific part called the antigenic determinant or epitope. The determinant in most protein Ag is formed by the sequence of 4-8 amino acid residues, and in polysaccharide Ag by 3-6 hexose residues. The number of determinants in each substance varies. For instance, there are at least 5 determinants in ovalbumin, a minimum of 8 in diphtheria toxin, and over 40 in thyreoglobulin. Ag are classified as exogenous (penetrating from outer environment), endogenous (autoantigens - products of "self" cells), and allergens (Ag promoting allergic reactions).
ANTIBODIES
Antibodies (Ab) are soluble proteins that are present in blood and other biological fluids and destined to recognize and neutralize Ag.
• Antibodies bind antigen. Ab reveals an important and unique property (different from TCR) to bind Ag as it penetrates the organism in its "native" form without preliminary metabolic "processing". Therefore Ab are the major factor of immediate protection of the organism (e.g. against strong poisons, snake and scorpion bites, bee stings, etc.).
• Specific antibody is synthesized only and exclusively by a single B-lym-phocyte clone. During differentiation each single B-lymphocyte and its mitotically generated daughter cells (B-lymphocyte clone) become capable of synthesizing only a single variant of Ab with a unique Ag-binding site. This phenomenon is defined as immunoglobulin clonality.