Antigen Antibody Reaction Questions and Answers

Answer: A

Correct answer: Via their hypervariable regions

The antigen-binding site is formed by the three hypervariable loops, or complementarity determining regions, of the heavy chain paired with the three of the light chain. Binding is entirely non-covalent - hydrogen bonds, electrostatic and hydrophobic interactions and van der Waals forces - which is what makes it reversible and why affinity varies so widely between antibodies.

Answer: D

Correct answer: Carboxy terminus of the heavy chain

The class is defined by the constant region of the heavy chain, and the constant region lies at the carboxy end. Sequencing from the amino terminus would give the variable domain, which differs between antibodies of the same class and is therefore no guide to it. The light chain carries no class information at all.

Answer: C

Correct answer: both (a) and (b)

Both tests read the neutralisation of a toxin by antibody. In Nagler’s reaction, antitoxin on part of an egg yolk plate prevents the lecithinase of Clostridium perfringens from producing opalescence there. In the Schick test, diphtheria toxin injected intradermally produces a reaction only in those without enough circulating antitoxin to neutralise it.

Answer: B

Correct answer: single diffusion in two dimensions

This is single radial immunodiffusion, the Mancini technique: antibody is incorporated into the gel and antigen is placed in a well, so only the antigen diffuses - hence single - and it spreads radially in two dimensions. The area of the ring of precipitate is proportional to the antigen concentration, which makes the method quantitative.

Answer: B

Correct answer: Antigens and antibodies are in optimal proportion

Lattice formation requires that antigen and antibody be present in roughly equivalent proportions, so that each multivalent antibody bridges two antigen molecules and the complex grows in three dimensions. In antibody excess every antigen site is saturated by separate antibody molecules, and in antigen excess every antibody is saturated by separate antigens - in both prozone and postzone the lattice cannot form and the reaction is falsely negative.

Answer: D

Correct answer: IgE

IgE is present in serum at only a few hundred nanograms per millilitre, five or six orders of magnitude below IgG. Most of it is not free in serum at all but bound to high affinity receptors on mast cells and basophils, which is exactly why so small a quantity can produce so violent a reaction.

Answer: A

Correct answer: indirect immunofluorescence

The FTA test fixes Treponema pallidum to a slide, adds the patient’s serum, and then detects any bound human antibody with a fluorescein-labelled anti-human globulin. Because the label is on the second antibody rather than on the first, it is an indirect method, and the extra layer amplifies the signal as well as allowing one labelled reagent to serve many tests.

Answer: B

Correct answer: T-cell receptors must interact with antigen uniquely presented by other cells but not with free antigen

This is the essential difference. An antibody binds free antigen in solution and recognises conformational shape; a T-cell receptor recognises only a short peptide held in the groove of a major histocompatibility complex molecule on another cell. That restriction is what confines T cells to surveying what other cells are displaying.

Answer: C

Correct answer: the variable region of the heavy chain remains the same but its constant region changes

This is what makes class switching useful: recombination replaces the heavy chain constant region gene while leaving the assembled variable region intact, so the antibody keeps the specificity that was selected for and acquires new effector functions. The light chain is not involved at all.

Answer: D

Correct answer: They are molecule with a single, defined amino acid sequence.

This is the false statement. Antibodies are enormously diverse: the variable domains are generated by the recombination of gene segments and further altered by somatic hypermutation, so a serum contains millions of different sequences. Only a monoclonal antibody, produced by a single clone, has one defined sequence. The other three statements are all true.