Q.14 A neonatally thymectomized mouse, immunized with protein antigen shows
- (A) Both primary and secondary responses to the antigen
- (B) Only primary response to the antigen
- (C) Delayed type hypersensitive reactions
- (D) No response to the antigen
Neonatal thymectomy removes the thymus shortly after birth, preventing T cell maturation. This impairs cell-mediated immunity and T-dependent B cell responses while sparing innate/T-independent humoral immunity. The MCQ tests this classic immunology principle.
Correct Answer: (B) Only primary response to the antigen
Thymectomized mice lack mature T cells, so they show only primary antibody response (IgM from B cells) to protein antigens but no secondary response (IgG, memory) requiring T cell help.Thymus Role in Adaptive Immunity
The thymus generates CD4+ helper T cells and CD8+ cytotoxic T cells. Neonatal thymectomy depletes these, leaving B1 cells and marginal zone B cells for T-independent responses. Protein antigens (T-dependent) trigger primary IgM via direct B cell activation, but class switching/memory needs T cell cytokines/CD40L.
Explanation of All Options
Each response type’s T cell dependency:
-
(A) Both primary and secondary responses to the antigen
Incorrect. Secondary responses require memory T/B cells absent post-thymectomy. -
(B) Only primary response to the antigen
Correct. Primary IgM response persists via T-independent pathways; no T cell help for secondary IgG/memory. -
(C) Delayed type hypersensitive reactions
Incorrect. DTH (Type IV) is CD4+ T cell-mediated; thymectomy eliminates this entirely. -
(D) No response to the antigen
Incorrect. B cells respond directly to protein antigens with low-affinity IgM primary response.
Option T Cell Requirement Post-Thymectomy Example (A) Both primary & secondary Yes (secondary) No (B) Only primary No Yes (C) DTH reactions Yes (CD4+) No (D) No response N/A No Experimental Context & Applications
Classic experiments (Miller 1961) showed thymectomized mice reject skin grafts poorly but produce primary antibodies. Modern relevance: SCID models, autoimmunity studies (thymectomy induces tolerance breakdown). For biotech, understand for vaccine design targeting T-independent antigens in immunocompromised hosts.
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