Largest immunoglobulin is

Appeared in: VSSC Nursing Officer - 2021

Explanation

IgM is the largest immunoglobulin due to its unique pentameric structure, which consists of five monomer units linked by a J-chain.
Its molecular weight is approximately 900,000-970,000 Daltons, far exceeding that of other immunoglobulin classes.
IgM is the first antibody produced during a primary immune response and is confined to the bloodstream due to its large size.

Option A: IgG is a monomer and is the most abundant immunoglobulin in the serum, but it is the smallest, with a molecular weight of about 150,000 Daltons.
Option B: IgA is found as a monomer in serum and as a dimer in secretions. Even in its larger dimeric form (approx. 385,000 Daltons), it is significantly smaller than the IgM pentamer.
Option D: IgD is a monomer with a molecular weight of about 180,000-185,000 Daltons. It is found in very low concentrations in the serum.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Characteristics and structure of immunoglobulins as background academic context rather than a clinical decision trigger.
A key nursing responsibility is interpreting lab results. Knowing that elevated IgM suggests an acute infection helps in prioritizing patient assessment and care for signs of infection.
In neonatal nursing, understanding that IgM does not cross the placenta is critical. A positive IgM test in a newborn is a red flag for a congenital infection, requiring immediate investigation and intervention.
What if? If a patient had a chronic autoimmune disease, you would expect to see elevated levels of IgG, not IgM, as IgG is associated with long-term (secondary) immune responses.

How to Approach the Question

This is a factual recall question. First, identify the core concept being tested: the physical properties of immunoglobulins.
Recall the basic structure of each immunoglobulin class: IgG, IgD, and IgE are monomers (single units); IgA is a dimer (two units); and IgM is a pentamer (five units).
Logically deduce that the structure with the most units (pentamer) will be the largest and heaviest.
Select IgM as the correct answer based on its pentameric structure.

Concept Tested & Keywords

QtBnuVugbAQvQxvwKK1c4V

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 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart B (pp 240-463 of 478) p. 92-94

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