ANTIGENS & ANTIBODIES.pptx

ANTIGENS & ANTIBODIES IMPORTANT DEFINITIONS Antigens are molecules that have the capacity to elicit the formation of antibodies and they react specifically with these antibodies. Immunogens are molecules that induce an immune response. Haptens are molecules that cannot induce an immune response but...

ANTIGENS & ANTIBODIES IMPORTANT DEFINITIONS Antigens are molecules that have the capacity to elicit the formation of antibodies and they react specifically with these antibodies. Immunogens are molecules that induce an immune response. Haptens are molecules that cannot induce an immune response but have the ability to react with specific antibody. They can also become immunogenic when coupled to a carrier molecule or protein. Antigen/Antibody reactions are highly specific and this property has been employed in the in the diagnosis of diseases/ identification of microorganisms. PROPERTIES OF A MOLECULE THAT INFLUENCE IMMUNOGENICITY • Foreignness • Molecular size • Chemical composition & Heterogeneity • Antigenic determinants (Epitope) • Dosage, Route & Timing of administration. • Genotype of the host ANTIBODIES • They are gamma globulin proteins (Immunoglobulins) • They are present on B-cell membrane and secreted by Plasma cells • They are the effector molecules humoral immunity & are present in the serum • They constitute about 20% of protein in the Plasma. STRUCTURE & TYPES OF IMMUNOGLOBULINS • IgA constitutes approx 15% of total Ig in serum • Has molecular weight (170,000-400,000) • Serum concentration of 60-400mg/dl • Major Ig in secretions (colostrum, saliva, tears) • Respiratory, Intestinal & Genital tract secretions • Secretory IgA in external secretions • Exists mainly as monomer in serum but polymeric forms occur IgG Approx 75% total Ig in serum Serum conc of 700-1,600 mg/dl Molecular weight 150,000 Most abundant class in serum Predominant antibody in secondary response • Opsonizes/activates complement • Only Ig to cross the placenta • Has 4 subclasses (IgG1, IgG2,IgG3,IgG4) • • • • • IgM • • • • • • • • Approx 9% of total serum Ig Molecular weight 900,000 Serum conc 40-230 mg/dl Occurs as monomer on B-Cell surface where it functions as binding receptor for antigens Pentamer in serum (5H2L2 units & J chain) First Ig in primary response First Ig to be synthesized by neonates Highest avidity/most efficient Ig in Complement fixation, agglutination etc IgE • Approx 0.004% of total Ig in serum • Molecular weight of 190,000 • Fc portion of IgE binds to the surface of mast cells and basophils • It mediates immediate hypersensitive reactions • Important in host defence against helminth or worm infections IgD Approx 0.2% of total serum Ig Molecular weight of 180,000 An early marker of B-Cell activation It functions as a B-Cell receptor and may participate in B-Cell maturation, maintenance and activation • That is, can have a regulatory role e.g to enhance protective antibody response of the IgM, IgG & IgA isotypes. • • • • Immunoglobulin Structure