Nude mice are not resistant to xenograft due to the absence of ________________

Explanation

## Correct Answer: C. T cell Nude mice (athymic mice) lack a functional thymus gland, resulting in severe T cell deficiency while retaining relatively intact B cell and NK cell populations. Xenograft rejection is primarily a **T cell-mediated (cellular) immune response**. The rejection occurs through two main mechanisms: (1) direct recognition of foreign MHC molecules on donor cells by host CD8+ cytotoxic T lymphocytes (CTLs), and (2) indirect recognition where host antigen-presenting cells process and present donor antigens to host CD4+ T helper cells, activating both cellular and humoral responses. Without functional T cells, nude mice cannot mount this critical cell-mediated rejection response, making them permissive hosts for xenografts. B cells alone (producing antibodies) and NK cells (innate immunity) are insufficient to reject xenografts in the absence of T cell help and coordination. This is why nude mice are routinely used in research to establish human tumor xenografts and study xenograft biology without immunological rejection complications. The absence of T cells is the discriminating factor that explains xenograft acceptance in these immunocompromised animals. ## Why the other options are wrong **A. Both B and T cell** — While T cell absence is the critical factor, nude mice retain functional B cells and NK cells. If both B and T cells were absent, the answer would be incomplete and misleading. The question specifically tests understanding that T cells are the PRIMARY mediator of xenograft rejection. Choosing this option suggests confusion about which arm of immunity is responsible for xenograft rejection. **B. NK cell** — NK cells are part of innate immunity and play a minor role in xenograft rejection compared to T cells. Nude mice retain functional NK cells despite lacking T cells, yet they still accept xenografts readily. This is an NBE trap—students may confuse NK cell importance in general immunosurveillance with their actual role in xenograft rejection, which is secondary to T cell-mediated responses. **D. B cell** — Nude mice retain functional B cells and antibody production. While B cells contribute to xenograft rejection through complement-dependent cytotoxicity and ADCC, they require T cell help (CD4+ T cells) for optimal antibody responses. B cell deficiency alone does not explain xenograft acceptance in nude mice, as T cell-mediated rejection is the dominant mechanism. ## High-Yield Facts - **Nude mice** are athymic (lack thymus) → severe T cell deficiency but normal B and NK cells - **Xenograft rejection** is primarily T cell-mediated (CD8+ CTLs and CD4+ Th cells) via direct and indirect MHC recognition - **T cell help** is essential for optimal B cell antibody responses; B cells alone cannot reject xenografts efficiently - **NK cells** contribute to innate immunity but are insufficient to reject xenografts without T cell coordination - **Clinical use**: Nude mice are preferred hosts for human tumor xenografts in cancer research because they accept foreign tissue without rejection ## Mnemonics **NUDE = No Thymus = No T cells** Nude mice → athymic → T cell deficiency. Remember: the thymus is the T cell factory. No thymus = no T cells. Use this when identifying why nude mice accept xenografts. **XenoREJECT = T cell REJECTs foreign MHC** Xenograft rejection requires T cells recognizing foreign MHC (direct) or processed antigens (indirect). T cells are the gatekeepers of xenograft immunity. B and NK cells are supporting players, not the main actors. ## NBE Trap NBE pairs "nude mice" with "immunodeficiency" to lure students into choosing "Both B and T cell" or "NK cell." The trap is forgetting that nude mice retain B and NK cells—the question specifically tests whether you know that T cells are the CRITICAL missing component for xenograft rejection, not a general immunodeficiency state. ## Clinical Pearl In Indian research institutions, nude mice are extensively used to establish patient-derived xenografts (PDX) from Indian cancer patients for drug testing and personalized oncology studies. The absence of T cell-mediated rejection allows human tumors to engraft and grow in these mice, enabling preclinical evaluation of novel therapeutics before human trials—a critical step in translational cancer research in India. _Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Ch. Immunology & Host Defenses); Robbins & Cotran Pathologic Basis of Disease (Ch. Neoplasia - Xenograft models)_