Antigen-Antibody Reactions
Nada Mohamed Ahmed ,
MD, MT (ASCP)i
OBJECTIVES
• Basic terminology in Antigen-antibody reactions.
• Types of Antigen-antibody reactions.
Basic terminology in Antigen-antibody reactions.
The affinity:
is the strength of the reaction between a single
antigenic determinant and a single combining
site on the antibody
Valence: the number of epitopes
Avidity: is the collective affinity of multiple
binding sites(affinity+ Valence)
Types of Antigen-antibody reactions:
– Precipitation
– Agglutination
– Neutralization (Antitoxins)
– Opsonization
– The complement activation
Membrane attack complex
Consequences of Antibody Binding
PRECIPITATION
 Is the reaction of soluble Ag with soluble Ab.
 The reaction results in the formation of Ag-Ab
complexes (lattices)
Antigen
Antibody
The Quantitative Precipitation Reaction:
 Varying amounts of Ag are mixed and incubated with
Constant volume of antisera
 Precipitate is measured, amount of precipitate depends
on :
o the ratio of Ag : Ab
o The Ab avidity
Plot in a curve, three zones are detected:
i. Zone of Ag excess : insufficient Ab  too small complexes
to precipitate
i. Equivalence zone : large lattice is formed  visible
precipitates
i. Zone of Ab excess : not enough Ag  too small complexes
to precipitate
AGGLUTINATION
 Abs can bind and cross-link cells or particles 
aggregate formation
 Entrap microbial invaders
 IgM & IgA are the most suitable (IgG in sufficient
amounts can agglutinate cells)
Agglutination
RBC
IgM Antibody
RBC
RBC
RBC
RBC
RBC
RBC
RBC
IgG Antibody
RBC
RBC
RBC
Applications of Agglutination
1.
Agglutination/Hemagglutination:
a. Qualitative agglutination test
Determination of blood types or antibodies to blood group Ags
b. Quantitative agglutination test
Agglutination tests can also be used to measure the level of antibodies to
particulate antigens.(titration)
2.
Passive hemagglutination: erythrocytes are coated with a soluble
antigen (e.g. viral antigen, a polysaccharide or a hapten) and use the
coated red blood cells in an agglutination test for antibody to the soluble
antigen
3- Coomb's Test (Antiglobulin Test)
4- ABO GROUPING
ABO blood grouping
Blood groups are :Group A : red blood cells contain A antigens and the
plasma has anti-B antibodies.
Group B : B antigens are found in the red blood cells and
anti-A antibodies in the plasma.
Group AB : the red blood cells have both A and B
antigens, however there are no antibodies.
Group O : this time the plasma contains both types of
antibodies but neither type of antigen.
Methods of ABO:Three manual methods can be used when
performing blood grouping:
- Slide method
- Test tube method
-
Slide or Tile Testing
This technique may be used for emergency ABO
grouping tests or for preliminary grouping
particularly in an outdoor camp, however it should
always be supplemented with a cell and serum
grouping using any one of the other above mentioned
techniques.
Disadvantages
- Less sensitive than the tube test
- Drying up of the reaction mixture can cause
aggregation of cells, giving false positive results.
- Weaker reactions are difficult to interpret.
Procedure:1. Place 1 drop of anti-A and 1 drop of anti-B reagent
separately on a labeled slide .
2. Add 1 drop of blood to each drop of the typing
antiserum
3. Mix the cells and reagent using a clean stick.
Spread each mixture evenly on the slide over an area
of 10-15 mm diameter.
4. Tilt the slide and leave the test for 2 minutes at
room temperature (22°-24°C). Then rock again and
look for agglutination.
5. Record the results.
Forward Grouping Reagent
17
Determination of the blood group in labs
Blood groups are determined using serological
tests:
NEUTRALIZATION
 Is the binding of Ab to microbial epitopes or
soluble molecules(e.g. toxins) which inhibits
their binding to host cells.
 Abs are mostly IgG & IgA
 Used to identify toxins and viruses
OPSONIZATION
Is the process by which a pathogen is marked
(tagged) for ingestion and destruction by
phagocytic cells