Antigen Antibody Reaction Questions and Answers

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.

Answer: A

Correct answer: antibodies

Agglutination involves particulate antigen - whole cells or coated particles - so very little antibody is needed to produce a visible clumping, and the test detects antibody at concentrations well below what precipitation requires. This is why so many diagnostic serological tests are agglutination tests, and why soluble antigens are deliberately coated onto latex or red cells to convert precipitation into agglutination.

Answer: D

Correct answer: all of these

This is the standard device for gaining sensitivity: adsorb the soluble antigen onto a particle and the reaction becomes an agglutination, visible with far less antibody. Latex beads, bentonite and red cells are all used as carriers, the choice depending on the antigen and on how the result is to be read. The specificity comes from the antigen; the particle only makes the reaction visible.

Answer: A

Correct answer: antibodies to lyse cells via the C8 and C9 components

C8 and C9 are the terminal components: C9 polymerises to form the pore of the membrane attack complex, and the cell lyses osmotically. Complement does also opsonise, through C3b, but the option naming C8 and C9 identifies the lytic function that these particular components perform, which is the one being asked about.

Answer: D

Correct answer: is determined by the heavy chain type

The five heavy chain types - mu, gamma, alpha, delta and epsilon - define IgM, IgG, IgA, IgD and IgE respectively. The light chain, kappa or lambda, is independent of class and is used to describe the antibody’s type rather than its class. Neither the antigen nor the major histocompatibility complex has any part in determining it.