**Question:** IgM have how many four-peptide subunits: March 2010

**Core Concept:** Immunoglobulins, also known as antibodies, are proteins produced by the immune system in response to an infection or other foreign substances. Immunoglobulin M (IgM) is one of the five classes of immunoglobulins and plays a crucial role in the immune response.

**Why the Correct Answer is Right:** IgM is composed of five identical polypeptide chains, each consisting of four polypeptide subunits. These subunits are arranged in a pentameric structure, forming the characteristic "Y" shape that is characteristic of IgM molecules. This arrangement provides IgM with a high affinity for antigens, enabling it to neutralize viruses and bind to multiple antigens simultaneously.

**Why Each Wrong Option is Incorrect:**

A. IgM is not composed of three or four polypeptide subunits; the correct number is five.
B. IgM does not have two polypeptide subunits; it has five identical polypeptide chains, each consisting of four subunits.
C. IgM does not have seven or eight polypeptide subunits; the correct number is five.
D. IgM does not have six polypeptide subunits; the correct number is five.

**Clinical Pearl / High-Yield Fact:** Understanding the structure and function of immunoglobulins, such as IgM, is essential for medical students, particularly during immunology and antibody-related topics in their curriculum. The pentameric structure of IgM allows for a high-affinity binding to antigens, contributing to the immune response against infections and other foreign substances.

**Correct Answer:** D. IgM have five four-peptide subunits.

**Explanation:** IgM is composed of five identical polypeptide chains, each consisting of four polypeptide subunits. These subunits are arranged in a pentameric structure, forming the characteristic "Y" shape that is characteristic of IgM molecules. This arrangement provides IgM with a high affinity for antigens, enabling it to neutralize viruses and bind to multiple antigens simultaneously, playing a crucial role in the immune response against infections and foreign substances.