Antigen-Antibody Reactions PDF

Antigen and Antibody Reactions Antigens Whole cells, viruses and complex molecules elicit the immune response. Small molecules HAPTEN, simple molecules ,when...

Antigen and Antibody Reactions Antigens Whole cells, viruses and complex molecules elicit the immune response. Small molecules HAPTEN, simple molecules ,when conjugated with e.g. protein can stimulate the immune system. 2 INTRODUCTION The antigens and the antibodies combine specifically with each other. This interaction between them is called Antigen-Antibody reaction. It may be abbreviated as Ag – Ab reaction. These form the basis for humoral immunity or antibody mediated immunity. These reactions form the basis for detection of infectious disease causing agents and also some non-specific Ag’s like enzymes. INTRODUCTION When Ag – Ab reactions occur in vitro, they are known as serological reactions. The reactions between Ag and Ab occur in three stages.  I n f ir st st a g e t h e r e a c t i o n i n v o l v e s formation of Ag-Ab complex.  The second stage leads to visible events like precipitation, agglutination etc.  The third stage includes destruction of Ag or its neutralization  Salient Features of Antigen – Antibody Reaction Specif ic it y of Ant igen – Ant ibody Reaction. Immune complex. Binding Site of Antigen – Antibody Reaction. Binding Force of Antigen – Antibody Reaction.  Specificity of Antigen – Antibody Reaction  Specificity refers to the ability of an individual antibody combining site to react with only one antigenic determinant or the ability of a population of antibody molecules Each antibody binds to a to react with only one specific antigen; an antigen. interaction similar to a lock and key.  Binding Site of Antigen – Antibody Reaction In antigen - antibody reaction, the antibody attaches with the antigen. The part of antigen which combines with antibody is called Epitope. A n e p i to p e , a l so k n o w n a s a n t i g e n i c determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope.  Binding Site of Antigen – Antibody Reaction  Antigen and Antibody to Show Epitope And Paratope  Binding Force of Antigen – Antibody Reaction The binding between antigen and antibody in Ag – Ab reaction is due to three factors namely:  Closeness between antigen and antibody.  Non – covalent bonds or Intermolecular forces.  Affinity of antibody.  Properties of Antigen – Antibody Reaction The properties of antigen and antibody can be explained with the help of three points. They are: Antibody Affinity. Antibody Avidity Cross reaction.  Affinity of Antibody  Affinity of antibody: Antibody affinity is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody.  Interactions between antigen and antibody involve non-covalent binding of an antigenic determinant (epitope) to the variable region (complementarity determining region, CDR) of both the heavy and light immunoglobulin chains  Avidity of Antibody It is th e stren gth of th e bon d after th e formation of Ag-Ab complexes. It is used to denote the overall capacity of antibodies to combine with the multivalent antigen. A multivalent Ag has many types of antigenic determinants. When injected into the blood, each antigenic determinant stimulates the production of a particular antibody.  Cross Reaction An antiserum raised against an Ag, can also react with a similar Ag of another type. This is called cross reaction and the Ag which produces the cross reaction is called Cross reactive Ag.  But the strength of Ab raised against its own Ag is strong  Types of Antigen – Antibody Reaction Precipitation Reaction. Agglutination Reaction. Complement Fixation. ELISA – Enzyme Linked ImmunoSorbent Assay. Immunofluorescence.  Precipitation Reaction  When a soluble Ag combines with its Ab in the presence of an electrolyte (NaCl) at a particular temperature and pH, it forms an insoluble precipitate of Ag-Ab complex.  The Ab causing precipitation is called Precipitin and the reaction is called as precipitation reaction. Antibodies Antigens Ag-Ab complex  Agglutination

 In this test, the antigen is particulate(e.g., bacteria and red blood cells)1or is an inert particle (latex beads) coated with an antigen.  When red cells are used, the reaction is called hemagglutination.  Antibody, because it is divalent or multivalent, cross-links the antigenically multivalent particles and forms a latticework, and clumping (agglutination) can be seen.  This reaction can be done in a small cup or tube or with a drop on a slide.  One very commonly used agglutination test is the test that determines a person’s ABO blood group  ELISA for antigen detection  Add the patient’s serum then wash  add specific antibodies to the antigen  wash  add 2° antibody linked with the enzyme then wash  add specific substrate.  read the reaction. P N ELISA for Antibody detection Antigen 1° Antibody in 2° antibody Chromogenic coating the patient's (enzyme- substrate serum linked) Indirect ELISA  Application of Antigen – Antibody Reaction  The chief use of antigen-antibody reactions are: Determination of blood groups for transfusion. Serological ascertainment of exposure to infectious agents. Development of immunoassays for the quantification of various substances. To detect the presence or absence of protein in serum. Determining the characteristics of certain immuno- deficiency disease.