Largest immunoglobulin is?

Appeared in: HPSSC Staff Nurse - 2021

Explanation

IgM is the largest immunoglobulin due to its pentameric structure, which consists of five monomer units linked by a J-chain.
This structure gives IgM a molecular weight of approximately 900 kDa, far exceeding the other immunoglobulin classes.
Because of its large size, IgM is mainly confined to the intravascular space.
It is the first antibody to be synthesized in a primary immune response, making it a key indicator of acute infection.

Option A: IgG is a monomer with a molecular weight of about 150 kDa. While it is the most abundant immunoglobulin in the serum, it is not the largest.
Option C: IgA exists as a monomer or a dimer. The dimeric form has a molecular weight of about 385-400 kDa, which is significantly smaller than the IgM pentamer.
Option D: IgE is a monomer with a molecular weight of about 190 kDa. It is present in the lowest concentration in the serum.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification and properties of immunoglobulins as background academic context rather than a clinical decision trigger.
The presence of specific IgM antibodies in a patient's serum is a key diagnostic marker for an acute or recent infection (e.g., IgM anti-Hepatitis A virus indicates acute hepatitis A).
Conversely, the presence of specific IgG antibodies typically indicates a past infection or immunity from vaccination.
What if? - If a newborn has elevated levels of IgM, it suggests an intrauterine infection (e.g., with rubella or cytomegalovirus), because maternal IgM cannot cross the placenta due to its large size. The fetus must have produced its own IgM in response to the infection.

How to Approach the Question

This is a factual recall question from immunology.
Identify the keyword in the question stem: "Largest".
Recall the basic properties of the five classes of immunoglobulins (IgG, IgA, IgM, IgD, IgE).
Focus on the structure of each immunoglobulin: IgG, IgD, and IgE are monomers; IgA is a monomer or dimer; IgM is a pentamer.
Conclude that the pentameric structure of IgM makes it the largest immunoglobulin by a significant margin.

Concept Tested & Keywords

QWlutddqgWoOc4i6DnGb5j

E6 Biochemistry U Satyanarayana— Part 2 (pp 421-820 of 840) p. 349-351

E6 Harper's Illustrated Biochemistry2023 (pp 26-793 of 813) p. 630-632

E6 Parks TextBook of Preventive & Social Medicine part 1 — Subpart A (pp 26-201 of 604) p. 101-103

Practise the full HPSSC Staff Nurse - 2021

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