In mice, mutations of which of the following genes cause the 'scurfy' phenotype (scaly skin and multiple autoimmune disorders)?

Correct Answer: A

Rationale: Rationale: 1. FOXP3 gene encodes a transcription factor crucial for regulatory T cells function. 2. Mutations in FOXP3 lead to dysfunctional regulatory T cells, causing autoimmune disorders. 3. Scurfy phenotype matches the symptoms of autoimmune disorders seen with FOXP3 mutations. Summary: - B (AIRE): AIRE mutations cause autoimmune polyendocrine syndrome, not scurfy phenotype. - C (NFkB): NFkB is a transcription factor involved in immune response but not linked to scurfy phenotype. - D (IRF): IRF is a regulator of interferon signaling, not directly associated with scurfy phenotype.

Correct Answer: A

Rationale: The correct answer is A because during clonal selection, lymphocytes specific to the pathogen are activated, leading to their proliferation and differentiation into effector cells. This results in an increase in the number of lymphocytes specific for the pathogen, enhancing the immune response. Choice B is incorrect because clonal selection does not involve the elimination of non-specific lymphocytes but rather the expansion of specific lymphocytes. Choice C is also incorrect because clonal selection does not change the antigen receptor specificities expressed by lymphocytes but instead focuses on expanding the population of lymphocytes with receptors specific to the pathogen. Therefore, the correct outcome of clonal selection is the increase in the number of lymphocytes specific for the pathogen that activated the response, making choice A the most accurate option.

Correct Answer: D

Rationale: The correct answer is D because all the options contribute to improving flu vaccines. A cytotoxic T cell response can help block viral spread, strain-specific neutralizing antibodies can prevent viral entry, and targeting less variable regions of HA with antibodies can improve recognition across strains. Each approach addresses different aspects of the immune response to enhance vaccine efficacy. Therefore, combining these strategies can lead to a more comprehensive and effective flu vaccine. Options A, B, and C alone are not sufficient as they only target specific aspects of the immune response, while option D encompasses a holistic approach for better vaccine success.

Correct Answer: A

Rationale: The correct answer is A because cytotoxic T lymphocytes (CTLs) kill target cells by recognizing specific peptide antigens presented on major histocompatibility complex (MHC) class I molecules. This recognition triggers the CTLs to release cytotoxic molecules that induce apoptosis in the target cells. Choice B is incorrect because CTLs do not die in the process of killing target cells; they survive to continue their immune response. Choice C is incorrect as CTLs do not kill target cells by activating complement, but rather through direct cell-cell contact. Choice D is incorrect because CTLs carry out their effector function in peripheral tissues where they encounter target cells, not specifically in secondary lymphoid tissues.

Correct Answer: B

Rationale: Positive selection is necessary to produce a T-cell repertoire capable of interacting with self MHC molecules. During positive selection in the thymus, T-cells that can recognize self MHC molecules are retained, ensuring self-tolerance and functionality. Negative selection eliminates self-reactive T-cells. Induction of anergy is a state of T-cell unresponsiveness and does not contribute to the development of a functional T-cell repertoire. Choice A is incorrect as only positive selection is necessary for self MHC recognition.