Which of the following is true about IgM?

Appeared in: AIIMS Patna NO - 2020

Explanation

IgM is the first antibody class to be synthesized by a neonate and the first to appear during a primary immune response to an antigen.
Its appearance in the serum signals a recent or ongoing infection, making it a crucial diagnostic marker.
As a pentamer, it has ten antigen-binding sites, which makes it highly effective at agglutinating (clumping) pathogens and activating the complement system.
This initial IgM response is later followed by the production of IgG, which provides long-term immunity.

Option A: This statement is false. Immunoglobulins, including IgM, are a cornerstone of the adaptive immune system found in all jawed vertebrates.
Option C: This is incorrect. IgM is one of the five major isotypes (or classes) of immunoglobulins in humans, along with IgG, IgA, IgD, and IgE. The term in the option, 'isotopes', is a misspelling of 'isotype'.
Option D: This statement is false. IgM is the largest antibody, with a molecular weight of approximately 900,000–970,000 Daltons because it exists as a pentamer (a structure of five joined units).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Properties of Immunoglobulin M (IgM) as background academic context rather than a clinical decision trigger.
In clinical diagnostics, measuring IgM levels is crucial for diagnosing acute infections. Elevated IgM specific to a pathogen (like rubella or hepatitis A virus) indicates a recent infection.
The absence of specific IgM but the presence of specific IgG suggests a past infection or vaccination, indicating long-term immunity.
What if? If a newborn has elevated IgM levels, it strongly suggests a congenital infection (e.g., TORCH infections). This is because maternal IgM is too large to cross the placenta, so the fetus must have produced its own IgM in response to an in-utero infection.

How to Approach the Question

This is a factual recall question. First, identify the core subject: the properties of the IgM antibody.
Read each option carefully and evaluate it against your knowledge of immunology.
Option A: 'not produced by vertebrates' - Recall that antibodies are a hallmark of the vertebrate immune system. This is false.
Option B: 'initial immune response' - This is a key, defining feature of IgM. This is true.
Option C: 'not an isotype' - Recall the five main isotypes (GAMED). IgM is the 'M'. This is false. Note the misspelling of 'isotype'.
Option D: 'smallest antibody' - Recall that IgM is a large pentamer, making it the largest antibody. This is false.

Concept Tested & Keywords

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Practise the full AIIMS Patna NO - 2020

Attempt every question from this paper in a timed mock, then review the full solution for each one.