**Core Concept:** Immunoglobulins, also known as antibodies, are a class of proteins that play a crucial role in the adaptive immune system. They are produced by B cells and are essential for defending the body against pathogens, such as bacteria, viruses, and other foreign substances.

**Why the Correct Answer is Right:** Immunoglobulins (Ig) are a diverse group of proteins involved in the immune response. In healthy individuals, the most abundant immunoglobulin is **IgG**, which makes up approximately **75%** of the total immunoglobulins in the blood. IgG is a crucial antibody that helps neutralize toxins, eliminate pathogens, and activate the complement system.

**Why Each Wrong Option is Incorrect:**

A. **IgM**: Although present, IgM constitutes only **20%** of total immunoglobulins in the blood. IgM is typically found in higher concentrations during the early stages of an immune response.
B. **IgA**: IgA accounts for **10%** of total immunoglobulins and is primarily found in mucosal surfaces like the respiratory and gastrointestinal tracts, protecting against pathogens in these areas.
C. **IgE**: IgE is the least abundant immunoglobulin, representing approximately **5%** of total immunoglobulins. IgE is crucial for the hypersensitivity reactions and allergy responses, but its percentage is less when compared to IgG.
D. **IgD**: IgD is the least known and understood immunoglobulin, with only **0.01%** of total immunoglobulins. It is a precursor to IgM and plays a role in the activation of B cells.

**Clinical Pearl:** Understanding the relative abundance of immunoglobulins is essential for understanding the immune response and its various stages. The correct sequence of immunoglobulins during immune responses includes IgM, IgD, and IgA, followed by IgG and IgE.

**Correct Answer:** IgG (75%)