Match the test in Group I with its application in Group IIGroup IGroup IIP. Oakley-Fulthorpe test1. IgM detectionQ. Limulus amoebocyte lysate test2. Determining antigen-antibody specificityR. Weil-Felix reaction test3. Endotoxin detectionS. Complement-fixation test4. Rickettsial infection diagnosis

The match is P-2, Q-3, R-4, S-1.The Oakley-Fulthorpe test is a double immunodiffusion in tubes, used to examine antigen-antibody reactions and their specificity.The Limulus amoebocyte lysate test, which clots in the presence of lipopolysaccharide, detects bacterial endotoxin.The Weil-Felix reaction uses Proteus antigens that cross-react with rickettsial antibodies, so it is used to diagnose rickettsial infections.The complement-fixation test detects antibodies that fix complement; IgM fixes complement most efficiently, which is the pairing the key adopts.Each of the other options misplaces at least one of the three unambiguous pairings: LAL with endotoxin, Weil-Felix with rickettsiae, and Oakley-Fulthorpe with antigen-antibody specificity.

Antigen Antibody Interactions Questions and Answers

Q: Which one of the following is the most sensitive immunoassay?

The most sensitive of these is the radioimmunoassay.A radioimmunoassay measures an antigen by its competition with a radiolabelled antigen for a limited amount of antibody. Because radioactivity can be counted very precisely, the method detects antigens at picogram levels, which is why Yalow and Berson's RIA was used to measure hormones such as insulin in blood.Immunoelectrophoresis and radial immunodiffusion rely on visible precipitin lines or rings, which form only at much higher (microgram) concentrations of antigen. Immunofluorescence is used mainly to locate antigens in cells and tissues and is not a quantitative assay of comparable sensitivity.

Q: Antibody titer refers to the :

Antibody titer is the highest dilution of antibody suitable to give a positive result in a test system.A serum is diluted serially, for example 1:10, 1:20, 1:40 and so on, and each dilution is tested in an assay such as agglutination, haemagglutination inhibition or ELISA. The titer is the last dilution still giving a detectable positive reaction, usually reported as its reciprocal, such as 320. It is a relative measure used to compare samples, for example to show a rise in antibody between two samples in an infection.It is not the absolute amount or the exact concentration of antibody, which need quantitative assays against a standard, and it is not avidity, the overall binding strength of a multivalent antibody.

Q: The affinity of an antibody can be determined by measuring :

Affinity is determined by measuring the amount of antibody bound at various antigen concentrations.Affinity is the strength of binding between one antibody binding site and one epitope, expressed as an association constant Ka. It is measured at equilibrium, classically by equilibrium dialysis: the antibody is exposed to a series of antigen (hapten) concentrations, the bound and free antigen are measured, and a Scatchard plot of the data gives the affinity constant and the number of binding sites.The antibody's concentration tells how much there is, not how tightly it binds. The valency is the number of binding sites, which affects avidity rather than affinity. Toxin neutralisation is a functional test that depends on several factors besides affinity.

Q: Which one is NOT an agglutination test ?

The Mantoux test is not an agglutination test.The Mantoux test is a skin test for tuberculosis: purified protein derivative (PPD) is injected into the skin, and a positive result is the induration produced 48 to 72 hours later by a delayed-type (type IV) hypersensitivity reaction. No particles are clumped.The other three rely on agglutination. The Widal test for typhoid detects antibodies that clump Salmonella Typhi O and H antigens. The Coombs test uses anti-human globulin to agglutinate red cells coated with antibody. Blood grouping identifies ABO and Rh antigens by agglutination of red cells with known antisera.

Q: In RIA; radiolabelling is done for :

In radioimmunoassay the Antigen is radiolabelled.In the competitive RIA developed by Yalow and Berson, a fixed amount of radiolabelled antigen (for example insulin labelled with iodine-125) competes with the unlabelled antigen in the sample for a limited amount of antibody. The more antigen there is in the sample, the less labelled antigen is bound, and the concentration is read from a standard curve.Labelling the antibody belongs to immunoradiometric assays (IRMA), a later variant, and labelled secondary antibodies belong to indirect techniques such as ELISA or immunoblotting with radioactive detection. The antigen-antibody complex is what is measured at the end, not what is labelled at the start.

Q: Enzyme labelled antibodies are not generally used for which of the following techniques ?

Enzyme-labelled antibodies are not generally used for FACS.Fluorescence-activated cell sorting detects and sorts single cells by the light they emit, so its antibodies are conjugated to fluorochromes such as FITC or PE, not to enzymes. An enzyme's coloured product would diffuse away and could not label individual cells as they pass the laser.ELISA, immunohistochemistry and Western blotting commonly use antibodies linked to horseradish peroxidase or alkaline phosphatase, which convert a substrate into a coloured, chemiluminescent or precipitating product.

Match the test in Group I with its application in Group II

Group I	Group II
P. Oakley-Fulthorpe test	1. IgM detection
Q. Limulus amoebocyte lysate test	2. Determining antigen-antibody specificity
R. Weil-Felix reaction test	3. Endotoxin detection
S. Complement-fixation test	4. Rickettsial infection diagnosis

A.P-2, Q-3, R-4, S-1
B.P-2, Q-1, R-4, S-3
C.P-3, Q-1, R-2, S-4
D.P-4, Q-3, R-2, S-1

Exam Year(s)

GATE Life Sciences - 2023,

Answer: A

The match is P-2, Q-3, R-4, S-1.

The Oakley-Fulthorpe test is a double immunodiffusion in tubes, used to examine antigen-antibody reactions and their specificity.
The Limulus amoebocyte lysate test, which clots in the presence of lipopolysaccharide, detects bacterial endotoxin.
The Weil-Felix reaction uses Proteus antigens that cross-react with rickettsial antibodies, so it is used to diagnose rickettsial infections.
The complement-fixation test detects antibodies that fix complement; IgM fixes complement most efficiently, which is the pairing the key adopts.

Each of the other options misplaces at least one of the three unambiguous pairings: LAL with endotoxin, Weil-Felix with rickettsiae, and Oakley-Fulthorpe with antigen-antibody specificity.

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Which one of the following is the most sensitive immunoassay?

A.Immunoelectrophoresis
B.Immunofluorescence
C.Radial immunodiffusion
D.Radioimmunoassay

Exam Year(s)

GATE Life Sciences - 2025,

Answer: D

The most sensitive of these is the radioimmunoassay.

A radioimmunoassay measures an antigen by its competition with a radiolabelled antigen for a limited amount of antibody. Because radioactivity can be counted very precisely, the method detects antigens at picogram levels, which is why Yalow and Berson's RIA was used to measure hormones such as insulin in blood.

Immunoelectrophoresis and radial immunodiffusion rely on visible precipitin lines or rings, which form only at much higher (microgram) concentrations of antigen. Immunofluorescence is used mainly to locate antigens in cells and tissues and is not a quantitative assay of comparable sensitivity.

Discuss in Forum

Antibody titer refers to the :

A.Absolute amount of specific antibody
B.Avidity of specific antibody
C.Concentration of specific antibody
D.Highest dilution of antibody suitable to give a positive result in a test system.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: D

Antibody titer is the highest dilution of antibody suitable to give a positive result in a test system.

A serum is diluted serially, for example 1:10, 1:20, 1:40 and so on, and each dilution is tested in an assay such as agglutination, haemagglutination inhibition or ELISA. The titer is the last dilution still giving a detectable positive reaction, usually reported as its reciprocal, such as 320. It is a relative measure used to compare samples, for example to show a rise in antibody between two samples in an infection.

It is not the absolute amount or the exact concentration of antibody, which need quantitative assays against a standard, and it is not avidity, the overall binding strength of a multivalent antibody.

Discuss in Forum

The affinity of an antibody can be determined by measuring :

A.Its concentration
B.The valency of antigen binding
C.The amount of antibody bound at various antigen concentrations
D.Its ability to neutralize toxins

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

Affinity is determined by measuring the amount of antibody bound at various antigen concentrations.

Affinity is the strength of binding between one antibody binding site and one epitope, expressed as an association constant K_a. It is measured at equilibrium, classically by equilibrium dialysis: the antibody is exposed to a series of antigen (hapten) concentrations, the bound and free antigen are measured, and a Scatchard plot of the data gives the affinity constant and the number of binding sites.

The antibody's concentration tells how much there is, not how tightly it binds. The valency is the number of binding sites, which affects avidity rather than affinity. Toxin neutralisation is a functional test that depends on several factors besides affinity.

Discuss in Forum

Which one is NOT an agglutination test ?

A.Widal test
B.Coombs test
C.Mantoux test
D.Blood group test

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The Mantoux test is not an agglutination test.

The Mantoux test is a skin test for tuberculosis: purified protein derivative (PPD) is injected into the skin, and a positive result is the induration produced 48 to 72 hours later by a delayed-type (type IV) hypersensitivity reaction. No particles are clumped.

The other three rely on agglutination. The Widal test for typhoid detects antibodies that clump Salmonella Typhi O and H antigens. The Coombs test uses anti-human globulin to agglutinate red cells coated with antibody. Blood grouping identifies ABO and Rh antigens by agglutination of red cells with known antisera.

Discuss in Forum

In RIA; radiolabelling is done for :

A.Antigen
B.Antibody
C.Antigen antibody complex
D.Secondary antibody

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

In radioimmunoassay the Antigen is radiolabelled.

In the competitive RIA developed by Yalow and Berson, a fixed amount of radiolabelled antigen (for example insulin labelled with iodine-125) competes with the unlabelled antigen in the sample for a limited amount of antibody. The more antigen there is in the sample, the less labelled antigen is bound, and the concentration is read from a standard curve.

Labelling the antibody belongs to immunoradiometric assays (IRMA), a later variant, and labelled secondary antibodies belong to indirect techniques such as ELISA or immunoblotting with radioactive detection. The antigen-antibody complex is what is measured at the end, not what is labelled at the start.

Discuss in Forum

Enzyme labelled antibodies are not generally used for which of the following techniques ?

A.FACS
B.Immunohistochemistry
C.ELISA
D.Western blot

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

Enzyme-labelled antibodies are not generally used for FACS.

Fluorescence-activated cell sorting detects and sorts single cells by the light they emit, so its antibodies are conjugated to fluorochromes such as FITC or PE, not to enzymes. An enzyme's coloured product would diffuse away and could not label individual cells as they pass the laser.

ELISA, immunohistochemistry and Western blotting commonly use antibodies linked to horseradish peroxidase or alkaline phosphatase, which convert a substrate into a coloured, chemiluminescent or precipitating product.

Discuss in Forum

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