Antibody Structure and Function Questions and Answers

Answer: B

Antibody-coated pathogens are recognised through the F\(_{C}\) receptor.

An antibody binds the pathogen with its Fab arms, leaving the Fc (constant) tail pointing outward. Phagocytes, NK cells, mast cells and other effector cells carry Fc receptors that bind this tail, which triggers opsonic phagocytosis or antibody-dependent cell-mediated cytotoxicity.

CD4 and CD8 are co-receptors on T cells that bind MHC class II and class I molecules. The IFN-γ receptor binds a cytokine. None of them recognises antibody bound to a microbe.

Answer: D

The most abundant serum immunoglobulin is IgG.

IgG makes up about three-quarters of the immunoglobulin in normal serum. It is the main antibody of the secondary immune response, it is the only class that crosses the placenta, and it neutralises toxins and opsonises microbes.

IgA is the main antibody of secretions (saliva, tears, milk) but is less abundant in serum. IgM is the large pentamer of the primary response, at around a tenth of the total, and IgD is present only in trace amounts, mainly on B cells.

Answer: B

The correct match is P-5 ; Q-4 ; R-2 ; S-1.

• IgM is secreted as a pentamer of five four-chain units, so it contains ten heavy and ten light chains (5).
• IgE binds Fcε receptors on mast cells and basophils and triggers allergic reactions (4).
• IgA, as a secretory dimer, is the predominant antibody in secretions such as saliva, tears, milk and mucus (2).
• MHC molecules display peptides to T cells, which is antigen presentation (1).

Item 3, crossing the placenta, belongs to IgG, which is not in the list. The other options misassign IgM, IgE and IgA, and the last gives MHC the ten-chain structure.

Answer: A

The isotype is decided by the constant region of the heavy chain only.

There are five kinds of heavy-chain constant region, gamma, mu, alpha, delta and epsilon, and they define IgG, IgM, IgA, IgD and IgE. The heavy-chain constant region also sets each class's effector functions, such as complement fixation by IgM or mast-cell binding by IgE. Class switching changes only this region while the antigen-binding site stays the same.

The light-chain constant region comes in two types, kappa and lambda, and either can pair with any isotype, so it plays no part. The variable regions of both chains form the antigen-binding site and decide specificity, not class.

Answer: C

The incorrect statement is IgD is the most abundant immunoglobulin.

IgG is the most abundant immunoglobulin in serum, about three quarters of the total. IgD is present only in trace amounts in serum; its main role is as an antigen receptor on the surface of mature naive B cells, alongside IgM.

The other statements are correct. Secreted IgM is a pentamer joined by a J chain. IgA in secretions such as saliva, tears and breast milk is a dimer. IgE binds to mast cells and basophils and triggers the release of histamine in allergic reactions.

Answer: C

A Fab fragment binds antigen.

Papain cleaves an IgG molecule above the hinge into two Fab (fragment, antigen-binding) pieces and one Fc (crystallisable) piece. Each Fab contains a whole light chain plus the VH and CH1 domains of a heavy chain, so it keeps one antigen-binding site.

Pepsin cleaves below the hinge and gives a single F(ab')2 fragment, not Fab. Separating heavy and light chains by reducing the disulphide bonds gives free chains, not fragments, and Fab contains a complete light chain, so 'lacks light chains' is wrong.