Antigen-Antibody Reactions PDF

Antigen- Antibody reactions Antigen: a molecule (or parts of one) that causes antibody generation (Immunoglobulin) The specific region on an antigen recognized by an antibody  Affinity  Combined strength...

Antigen- Antibody reactions Antigen: a molecule (or parts of one) that causes antibody generation (Immunoglobulin) The specific region on an antigen recognized by an antibody  Affinity  Combined strength of total non-covalent interactions between single Ag- binding site of Ab and single epitope is affinity of Ab for that epitope.  Low affinity Ab: Bind Ag weakly and dissociates readily.  High affinity Ab: Bind Ag tightly and remain bound longer.  Avidity  Strength of multiple interactions between multivalent Ab and Ag is avidity. Avidity is better measure of binding capacity of antibody than affinity. High avidity can compensate low affinity.  Cross reactivity  Antibody elicited by one Ag can cross react with unrelated Ag if they share identical epitopeor have similar chemical properties.  Immune complex formation:  Ab have two Ag binding sites and most Ag have at least two epitopes, cross linking can occur producing large aggregates called immune complex.  If the Ag s are soluble molecules and the compex becomes large enough to settle out of soution a precipitation occurs  When the immune complex involves the crosslinking of cells or particles an agglutination reaction occurs Multivalent antigens combine with bivalent antibodies in varying proportions, depending on the antigen-antibody ratio in the reacting mixture.  Precipitation results with the formation of large lattice consisting of alternating antigens and antibody molecules (Mancini) This is a standard method for quantitative estimation of Ab. Slide test WIDAL test Tube agglutination test to detect bacterial infection Applications of Agglutination Reactions  Cross-matching and grouping of blood.  Identification of Bacteria. E.g. Serotyping of Vibrio cholera, Serotyping of Salmonella typhi and paratyphi.  Serological diagnosis of various diseases. E.g Rapid plasma regains (RPR) test for Syphilis, Antistreptolysin O (ASO) test for rheumatic fever.  Detection of unknown antigen in various clinical specimens. E.g. detection of Vi antigen of Salmonella typhi in the urine.  Complement fixation  A group of serum proteins collectively called complement.  During most antigen- antibody reactions, complement binds to the antigen-antibody complex and is used up, or fixed.  This process of complement fixation can be used to detect very small amounts of antibody.  Antibodies that do not produce a visible reaction, such as precipitation or agglutination, can be demonstrated by the fixing of complement during the antigen- antibody reaction.  Complement fixation  Complement fixation was once used in the diagnosis of syphilis (Wassermann test) and is still used to diagnose certain viral, fungal, and rickettsial diseases.  The complement-fixation test requires great care and good controls, one reason the trend is to replace it with newer, simpler tests.  The test is performed in two stages: complement fixation and indicator ` http://highered.mheducation.com/sites/0072556781/student_view0/chapter31/animation _quiz_4.html ELISA  TYPES OF ELISA 1 Non competitivebinding assay  A. Sandwich or Direct ELISA:  Antigen measuring system [Titrewellscoated with antibodies ; Enzyme labelled antibodies]  B. Indirect ELISA:  Antibody measuring system [Titrewellscoated with antigens ; Enzyme labelled antiantibodies] 2. Competitive binding assay [Titrewellscoated with antibodies ; Enzyme labelled antigens]  IMMUNOFLUORESCENCE:  Fluorescent dyes (fluoresceinor rhodamine) attached to known specific Abs, used to detect presence of Abs in serum or Ag (microorganisms) in a sample  Direct fluorescent Abtest: used to detect Ag or microorganism  Indirect fluorescent Abtest: used to find a specific Abin the serum  Quantitative Immunofluorescence-flow cytometry