Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Which class of antibody is first made by developing B cells inside bone marrow?
Options:
(A) IgG
(B) IgE
(C) IgA
(D) IgM
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.14)**
**1. Governing Concept**:
- **Topic**: Immunology & B-Cell Ontogeny (Immunoglobulin Isotypes)
- During B-lymphocyte development in the bone marrow:
- Pro-B cells rearrange heavy chain variable ($V_H-D_H-J_H$) gene segments.
- Successful $V_H-D_H-J_H$ joining pairs with the first constant region gene cluster, which is $C\mu$.
- Immature B-cells express membrane-bound **Immunoglobulin M (IgM)** on their surface as the antigen receptor (BCR).
- Later, alternative RNA splicing yields co-expression of IgM and IgD on mature naive B-cells.
- Hence, the first class of antibody synthesized by developing B-cells inside bone marrow is **IgM**.
**2. Option Analysis**:
- **Option (A) [IgG] is INCORRECT**: Produced following activation-induced class switch recombination in peripheral lymphoid tissue.
- **Option (B) [IgE] is INCORRECT**: Produced after class switching in allergic/parasitic responses.
- **Option (C) [IgA] is INCORRECT**: Mucosal class-switched antibody.
- **Option (D) [IgM] is CORRECT**: The primary and earliest immunoglobulin isotype expressed by developing B-cells.
**3. Final Answer**:
- **Correct Choice**: **D**
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