Q.45 Antibody formed immediately after infection is-
1. IgM
2. IgG
3. IgD
4. IgE
Title: First Antibody After Infection: IgM Primary Response Explained (MCQ Solved)
The antibody formed immediately after infection is IgM, the body’s first line of defense in the primary immune response.
Correct Answer: 1. IgM
IgM is the first antibody produced during an initial encounter with a pathogen, appearing within days as a pentamer for high avidity binding. It activates complement, opsonizes invaders, and peaks early before class-switching to other isotypes like IgG occurs in secondary responses.
Antibody Response Timeline
Primary response: Naive B cells differentiate into plasma cells secreting IgM first (days 3-7 post-infection). IgM levels rise rapidly due to its pentameric structure but decline as IgG (more stable, crosses placenta) dominates later.
Explanation of All Options
| Option | Role | Why Correct/Incorrect |
|---|---|---|
| 1. IgM | Primary response antibody; first secreted post-infection. | Correct: Pentameric, activates complement early. |
| 2. IgG | Most abundant in serum; secondary response dominant. | Incorrect: Appears later (~7-14 days); provides long-term immunity. |
| 3. IgD | B-cell receptor; minimal secreted form. | Incorrect: Regulatory role; not major post-infection antibody. |
| 4. IgE | Allergic/parasite defense; low serum levels. | Incorrect: Produced much later for hypersensitivity. |
