The vascular instability observed in severe sepsis is caused by

Correct Answer: D

Rationale: The correct answer is D because all the choices contribute to the vascular instability observed in severe sepsis. A: Systemic release of cytokines like TNF-α and IL-1β leads to vasodilation and increased capillary permeability. B: Complement activation causes inflammation and vasodilation. C: Release of high levels of lipid mediators like prostaglandins and leukotrienes further exacerbates vasodilation and vascular permeability. Therefore, all these mechanisms combined lead to the vascular instability seen in severe sepsis.

Correct Answer: C

Rationale: The correct answer is C because inducing antibodies that can recognize the less variable regions of the Hemagglutinin (HA) protein across different strains allows for cross-protection against various flu strains. This approach is effective as it targets conserved regions of the virus, increasing the likelihood of providing broad immunity. Option A focuses on cytotoxic T cell response, which primarily targets infected cells and may not prevent viral entry into uninfected cells. Option B, on the other hand, is limited to highly strain-specific neutralizing antibodies, which may not provide adequate protection against evolving flu strains. Therefore, option C is the most effective in producing better flu vaccines by targeting less variable regions of HA for broader immunity.

Correct Answer: D

Rationale: The correct answer is D: Allowed to mature and enter the periphery. In this scenario, since the developing B cell has no binding affinity for self-antigens in the bone marrow, it will not undergo clonal deletion or anergy. Therefore, it will be allowed to mature and enter the periphery where it can encounter foreign antigens and participate in the immune response. Choices A and B are incorrect because clonal deletion and anergy are mechanisms used to eliminate or inactivate self-reactive B cells to prevent autoimmunity. Choice C is incorrect because a B cell without self-binding affinity is not a cause of autoimmunity in the periphery.

Correct Answer: B

Rationale: The correct answer is B, germline gene rearrangement. T cell receptor and B cell receptor genes both undergo germline gene rearrangement during lymphocyte development, where gene segments are rearranged to generate diverse antigen receptor specificities. Somatic hypermutation and isotype switching are processes specific to B cells in the adaptive immune response, not T cells. Therefore, choices A and C are incorrect. Choice D is incorrect as well since somatic hypermutation is not a process involved in T cell receptor gene rearrangement.

Correct Answer: C

Rationale: The correct answer is C because smallpox was indeed introduced to the Americas by European explorers, leading to devastating effects on the Native populations who had no immunity. This weakened the Natives' ability to fight back against the Europeans. Option A is incorrect as there was no ongoing smallpox epidemic in the Americas before the arrival of the Europeans. Option B is also incorrect as there is no evidence that the Native populations used a lethal form of vaccination against smallpox. Option D is incorrect as only option C accurately explains the impact of smallpox on the Native populations in relation to European conquest.