Question:
Read the following statements about immunoglobulin M (IgM)
(A) IgM is the initial antibody produced during immune response to a foreign pathogen.
(B) IgM is a pentamer joined by J-chain with 10 F(ab) fragments and 10 paratopes
(C) Very high IgM levels are found in a group of primary immunodeficiencies characterized by susceptibility to infections and malignancies
(D) IgM cross placeta from mother to fetus and thus protects newborn progeny from intrauterine infections
Choose the correct answers from the options given below:
Read the following statements about immunoglobulin M (IgM)
(A) IgM is the initial antibody produced during immune response to a foreign pathogen.
(B) IgM is a pentamer joined by J-chain with 10 F(ab) fragments and 10 paratopes
(C) Very high IgM levels are found in a group of primary immunodeficiencies characterized by susceptibility to infections and malignancies
(D) IgM cross placeta from mother to fetus and thus protects newborn progeny from intrauterine infections
Choose the correct answers from the options given below:
(A) IgM is the initial antibody produced during immune response to a foreign pathogen.
(B) IgM is a pentamer joined by J-chain with 10 F(ab) fragments and 10 paratopes
(C) Very high IgM levels are found in a group of primary immunodeficiencies characterized by susceptibility to infections and malignancies
(D) IgM cross placeta from mother to fetus and thus protects newborn progeny from intrauterine infections
Choose the correct answers from the options given below:
Show Hint
IgM is a massive pentameric antibody (molecular weight \(\approx\) 900 kDa), which makes it physically impossible for it to cross any placental barrier.
IgG is the only antibody class capable of crossing the placenta.
IgG is the only antibody class capable of crossing the placenta.
Updated On: Jul 23, 2026
- (A), (B) and (D) only.
- (A), (B) and (C) only.
- (B), (C) and (D) only.
- (A), (C) and (D) only.
Show Solution
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
Immunoglobulin M (IgM) is one of the five major classes of immunoglobulins produced by B lymphocytes.
It plays a critical role in the primary immune response and possesses distinct structural and functional properties due to its polymeric nature.
Step 2: Detailed Explanation:
Let us analyze each statement individually:
Statement (A) is correct. IgM is the first antibody isotype secreted by plasma cells during the primary immune response to a novel antigen, prior to class switching to IgG, IgA, or IgE.
Statement (B) is correct. In its secreted form, IgM exists as a pentamer composed of five monomeric units joined together by disulfide bonds and a polypeptide J-chain (joining chain).
Each monomer has two antigen-binding sites, meaning the pentameric IgM has 10 Fab (antigen-binding) fragments and 10 paratopes, giving it a theoretical valence of 10.
Statement (C) is correct. A group of primary immunodeficiencies known as "Hyper-IgM syndromes" is characterized by defective class-switch recombination.
In these conditions, B cells cannot switch from producing IgM to other isotypes like IgG or IgA, leading to abnormally high levels of IgM in the blood, and making patients highly susceptible to pyogenic infections and malignancies.
Statement (D) is incorrect. Due to its large pentameric structure and high molecular weight (approximately 900,000 Daltons), IgM is too large to cross the placental barrier.
Only monomeric IgG can cross the placenta to provide passive immunity to the fetus, and this only occurs in species with specific placental types (such as hemochorial placentas in primates).
In domestic animals with epitheliochorial placentas (such as horses, pigs, and ruminants), no immunoglobulins can cross the placenta, and newborns must receive colostrum.
Therefore, statements (A), (B), and (C) are correct, while (D) is incorrect.
Step 3: Final Answer:
The correct option is (B).
Immunoglobulin M (IgM) is one of the five major classes of immunoglobulins produced by B lymphocytes.
It plays a critical role in the primary immune response and possesses distinct structural and functional properties due to its polymeric nature.
Step 2: Detailed Explanation:
Let us analyze each statement individually:
Statement (A) is correct. IgM is the first antibody isotype secreted by plasma cells during the primary immune response to a novel antigen, prior to class switching to IgG, IgA, or IgE.
Statement (B) is correct. In its secreted form, IgM exists as a pentamer composed of five monomeric units joined together by disulfide bonds and a polypeptide J-chain (joining chain).
Each monomer has two antigen-binding sites, meaning the pentameric IgM has 10 Fab (antigen-binding) fragments and 10 paratopes, giving it a theoretical valence of 10.
Statement (C) is correct. A group of primary immunodeficiencies known as "Hyper-IgM syndromes" is characterized by defective class-switch recombination.
In these conditions, B cells cannot switch from producing IgM to other isotypes like IgG or IgA, leading to abnormally high levels of IgM in the blood, and making patients highly susceptible to pyogenic infections and malignancies.
Statement (D) is incorrect. Due to its large pentameric structure and high molecular weight (approximately 900,000 Daltons), IgM is too large to cross the placental barrier.
Only monomeric IgG can cross the placenta to provide passive immunity to the fetus, and this only occurs in species with specific placental types (such as hemochorial placentas in primates).
In domestic animals with epitheliochorial placentas (such as horses, pigs, and ruminants), no immunoglobulins can cross the placenta, and newborns must receive colostrum.
Therefore, statements (A), (B), and (C) are correct, while (D) is incorrect.
Step 3: Final Answer:
The correct option is (B).
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