Immunology: avidity of IgM: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

Try it

Microbiology · Immunology: avidity of IgM

Sample item

Early in an infection, IgM antibodies with relatively low affinity at each binding site still agglutinate bacteria very efficiently. Which property explains this?

ASomatic hypermutation has increased the affinity of each siteBThe J chain increases the affinity of the variable regionsCMultiple binding sites on the pentamer give high overall avidityDThe Fc region binds the antigen directly

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Multiple binding sites on the pentamer give high overall avidity

A pentameric IgM has ten antigen-binding sites, and simultaneous binding to repeating epitopes gives high functional strength, termed avidity, even when each site has low affinity. Early IgM has had little somatic hypermutation. The J chain joins the monomers and has no role in antigen binding.

Option by option

A. Somatic hypermutation has increased the affinity of each site
Affinity maturation occurs later, after germinal centre reactions, and early IgM has had little of it.
B. The J chain increases the affinity of the variable regions
The J chain links the subunits and does not alter variable region affinity.
C. Multiple binding sites on the pentamer give high overall avidity (correct)
Correct. Multivalent binding raises avidity beyond the affinity of any one site.
D. The Fc region binds the antigen directly
Antigen is bound by the Fab variable regions, and the Fc region mediates effector functions.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Multiple binding sites on the pentamer give high overall avidity. A pentameric IgM has ten antigen-binding sites, and simultaneous binding to repeating epitopes gives high functional strength, termed avidity, even when each site has low affinity.

Why is "Somatic hypermutation has increased the affinity of each site" not the answer?

Affinity maturation occurs later, after germinal centre reactions, and early IgM has had little of it.

Why is "The J chain increases the affinity of the variable regions" not the answer?

The J chain links the subunits and does not alter variable region affinity.

Why is "The Fc region binds the antigen directly" not the answer?

Antigen is bound by the Fab variable regions, and the Fc region mediates effector functions.

What concept does this question test?

It tests Immunology: avidity of IgM in Microbiology. Early IgM has had little somatic hypermutation. The J chain joins the monomers and has no role in antigen binding.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Immunology: avidity of IgM?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.