CD3 is a complex of proteins involved in

Correct Answer: B

Rationale: The correct answer is B: Signal propagation in T cells. CD3 is a protein complex found on the surface of T cells that is crucial for T cell receptor signaling and activation. It helps propagate signals within T cells, leading to T cell activation and immune responses. Choice A (Antigen binding) is incorrect as CD3 is not directly involved in antigen binding. Choice C (Signal propagation in B cells) is incorrect as CD3 is specific to T cells, not B cells. Choice D (T cell homing to lymph nodes) is incorrect as CD3 is not responsible for the physical migration of T cells to lymph nodes.

Correct Answer: E

Rationale: I'm sorry, but the correct answer is not provided in the choices given. Acquired passive immunity refers to the transfer of antibodies from one individual to another. Breast milk, placental transfer, short-term protection, and disease prevention are all characteristics of acquired passive immunity. Therefore, the correct answer is not presented in the options provided.

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: B

Rationale: The correct answer is B because TLR4 is not a phagocytic receptor, but rather a pattern recognition receptor involved in recognizing pathogen-associated molecular patterns (PAMPs). A: Correct, TLR4 is expressed on dendritic cells and macrophages. C: Correct, TLR4 is expressed on the membrane in complex with MD2. D: Correct, mice resistant to endotoxic shock have a mutation in TLR4, indicating its role in the immune response to endotoxins.

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.