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Immunohaematology Reactions

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2. Immunohaematology Reactions

Antigen-Antibody Reactions

Different forms of observable antigen-antibody reactions which can occur in blood group serology are as follows
1. Agglutination
2. Sensitization
3. Haemolysis
4. Neutralization (Inhibition)
5. Precipitation
6. Complement fixation
7. Immunofluorescence
8. Radioimmunoassary (RIA)
9. Enzyme linked immunosorbant assay (ELISA)

The most commonly observed reactions in immunohaematology are

1. AgglutInation

It is defined as clumping of particles or red cells that have antigens corresponding to the antibody on their surface.
It is ‘the most common type of antigen-antibody reaction seen in blood group serology and forms the common end point of most of the tests involving red cell antigens and antibodies.
It occurs in two stages.
a. Attachment of antibody (sensitization)
b. Agglutination (as a result of collision between the sensitized red cells, cross-linking of cells occurs by bridges of antibody molecules leading to a lattice formation).

2. Sensitization

It is defined as coating or binding of antibody on the red cells surface without bringing about agglutination in saline. Since there is no visible phenomenon such as agglutination or haemolysis which can be observed in this reaction, an alternate method is used to demonstrate that an antigenantibody reaction has taken place.

3. Haemolysis

Many antibodies when they react with an antigen, activate complement and bind it on to the red cell. This may cause haemolysis of the red cells which is recognizable by the haemoglobin tinged supernatant obtained after centrigugation of the test mixture. Red cell lysis indicates the presence of antigen-antibody reaction and consumption of complement.

For the reaction to take place, the serum has to be fresh because complement rapidly denatures in a collected blood sample, IgM antibodies are predominantly complement fixing, rarely IgG antibodies may also bind complement. The optimal temper&ture for haemolysis is 37°C and it can best be demonstrated in serum as compared to plasma.

4. Neutralization (Inhibition)

Soluble form of blood group substances (antigens) have the effect of neutralizing (or inhibiting) the reaction of the corresponding blood group antibody. For a neutralization reaction, soluble antigens are added to the serum containing the specific antibody. If the activity of the antibody decreases or if it disappears completely, an antigen-antibody reaction can be assumed to have taken place. This test is most often used in testing saliva for A,B and H substances to determine the secretor status.

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