Immunology Antigen-Antibody Reactions Question And Answers

Antigen-Antibody Reactions

Question 1. Name antigen-antibody reaction, giving examples of each.
Answer:

When specific antigen and antibody molecules are mixed to- gether in solution, an antigen reacts with antibody and forms a complex. This formation of complex is due to combination of specifi antigen with specific antibody and is known as antigenantibody reaction.

Antigen-antibody reactions are useful in laboratory diagnosis
of various diseases and in identifiation of infectious agent.

Names of the antigen-antibody reaction are as follows:

1. Precipitation
2. Agglutination
3. Complement fiation test.

1. Precipitation: When a soluble antigen combines with its antibody in the presence of electrolyte at suitable temperature and pH, the antigen-antibody complex forms insoluble precipitation, example. VDRL test for syphilis (slide test), Kahn test for syphilis (tube flcculation test)

2. Agglutination reaction: When a particular antigen is mixed with its antibody in the presence of electrolytes at a suitable temperature and pH, then the particles are clumped and agglutinated. It is more sensitive than precipitate for the detection of antibodies, for example. WIDAL test for typhoid, Hemoagglutination test for blood grouping and typing, and Coombs’ test for detection of incomplete antibodies.

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3. Complement fiation test: It is a very sensitive test, for example Testfor kala-azar, amoebiasis, viral disease.

Question 2. Write a short note on prozone phenomenon and its clinical implications.
Answer:

An undiluted serum specimen with very high antibody titer does not show agglutination with the antigen. This practical problem is called as prozone effect.

• It occurs due to concentration of the antibody, coats on the indicator particles (red cells or latex) which carry the antigen on the surface. This coating prevents the particles from lining to each other.
• By diluting the serum appropriately, the antibody concentration is reduced to get the optimal proportion between the antigen and antibody molecules and thus the development of agglutination or flocculations.
• Prozone is important in some serological reaction when antibody is present in very high titre in serum.
• If the test is preformed by serial dilution the antibody and adding a constant amount of antigen the few tubes may not show precipitation due to excess of antibody molecules.
• Therefore if suffiently wide range of antibody dissolution is not used it may be wrongly intermittd as negative test.

Clinical Implementations

It is clinically implemented in

• Pregnancy
• VDRL test
• ELISA.

Question 3. Write a short note on agglutination reactions.
Answer:

• Agglutination reaction is a reaction between particulate antigen (agglutinogen) such as bacteria with its antibody (agglutinin) in the presence of electrolyte (NaCl) at a suitable temperature and pH which resulting in clumping or agglutination of particulate antigen.
• It is more sensitive for detection of antibodies.
• Agglutination reaction is more sensitive than precipitation reaction for detection of the antibody.
• Agglutination reaction occurs better with IgM antibody as compared to IgG antibody.

Mechanism of Agglutination Reaction

Lattce hypothesis proposed by Marrack proves the mechanism of agglutination reaction. According to this hypothesis an antibody molecule which is divalent form bridge between two antigen molecules. Antigen being multivalent combine by number of antibody molecules. This combination leads to the formation of multimolecular lattce which makes the reaction visible.

This is known as Lattce hypothesis. Hypothesis requires that.

• Antibody should be divalent
• Antigen and antibody should be in optimum proportion

Agglutination reaction occurs when antigen – antibody reacts in equivalent proportions. Lattce formation does not occur in zone of antigen-antibody excess.

Uses:

• For bacterial identifiation, i.e. serotyping of Shigella and Salmonella
• Hemagglutination test, i.e. Paul Bunnell test and Rose Waaler test
• Serological diagnosis of infection, i.e. widal test for typhoid
• For detection of incomplete antibodies, example ,Coombs test.