Antigen Antibody Reaction Questions and Answers

Answer: A

The example of passive agglutination is the latex agglutination test.

In passive agglutination a soluble antigen (or antibody) is coated onto inert carrier particles such as latex beads, so that when the matching antibody binds, the particles clump visibly. Latex tests for rheumatoid factor and C-reactive protein work this way.

A precipitation test uses soluble antigen without carriers and gives a precipitate, not agglutination. ELISA detects binding with an enzyme-linked colour reaction. The Coombs test is an antiglobulin test in which the antigens sit naturally on red cells, so it is not passive agglutination.

Answer: A

ELISA is based on immunochemical principles of antigen-antibody reaction.

In an enzyme-linked immunosorbent assay, an antibody binds its specific antigen, and an enzyme linked to one of them converts a colourless substrate into a coloured product. The colour, read by a spectrophotometer, measures how much antigen or antibody is present. It combines an immunological binding with a chemical (enzyme) reaction, hence immunochemical.

The detection is not physical, which rules out the second option. Radioactive labels are used in radioimmunoassay, not ELISA. 'Immunological reactions' is too general, since it covers agglutination and precipitation tests as well.

Answer: A

The keyed answer is HIV.

ELISA is the standard screening test for HIV infection: it detects antibodies to HIV (and, in newer kits, the p24 antigen) in the blood, and India's testing strategy uses ELISA or rapid tests in sequence to confirm a positive result. It is also used to screen donated blood for HIV.

ELISA is used for other infections too, such as hepatitis B surface antigen, so viral hepatitis is not a disease where ELISA is unknown. But the disease with which the test is most closely identified, and the one the key names, is HIV. Rabies is diagnosed mainly by antigen detection in tissue, and influenza by PCR or rapid antigen tests.

Answer: B

The zone of equivalence is the zone where antigen and antibodies are in optimum concentration.

When antigen and antibody are mixed in optimal proportions, they cross-link into a large lattice and the precipitation (or agglutination) is greatest. That region of the curve is the zone of equivalence.

With antibody in excess, the prozone, and with antigen in excess, the postzone, lattices do not form well and the reaction may appear negative. Equivalence describes the proportions of the two reactants, not whether the reaction has finished.

Answer: D

Correct answer: they mediate allergic reaction

Immediate allergic reactions are mediated by IgE, not IgM. The other three statements are all true of IgM: it is the most efficient complement fixer because its pentameric form presents several Fc regions together, it appears as a monomer on the B-cell surface as part of the antigen receptor, and it dominates the primary response before class switching occurs.

Answer: A

Correct answer: Usually with booster immunizations, going from IgM to IgG

Class switching happens as the response matures, particularly on re-exposure: the first encounter produces mainly IgM, and subsequent boosters produce predominantly IgG. The switch is driven by T-cell help and cytokines, and it recombines a different heavy chain constant region gene into place while leaving the specificity untouched.

Answer: B

Correct answer: antiviral proteins

Interferons are cytokines released by virus-infected cells that induce an antiviral state in neighbouring cells, switching on enzymes that degrade viral RNA and block translation. They act on the cell rather than on the virus, and they are not specific to any one virus, which is what makes them an early and general defence rather than an adaptive one.

Answer: C

Correct answer: present antigen for recognition by the T-cell antigen receptor

Both classes of major histocompatibility complex molecule are peptide-display platforms. Class I presents peptides from cytosolic proteins to CD8 T cells, class II presents peptides from endocytosed material to CD4 T cells. A T-cell receptor cannot see free antigen at all, which is why this presentation function is indispensable.

Answer: C

Correct answer: They will cross the placenta and fix complement.

This is the false statement, and it is false twice over: IgG is the only class transported across the placenta, and IgE does not activate the classical complement pathway. The other three are correct - IgE mediates immediate hypersensitivity, contributes to defence against helminths, and triggers histamine release from mast cells and basophils.

Answer: D

Correct answer: An antibody contains protein as its major chemical component and its synthesis may be elicited by the administration of a foreign protein or polysaccharide.

This option gets both halves right: an antibody is chiefly protein, and its production is provoked by a foreign antigen, which may be a protein or a polysaccharide. The others fail on a point of fact - there is no RNA in an antibody, it separates from albumin on electrophoresis, and its four chains are two identical pairs rather than four identical subunits.