Antigen Antibody Interactions Questions and Answers

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.

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.

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.

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.