In the 1960s, smallpox was endemic:

Correct Answer: A

Rationale: The correct answer is A: In many developing countries. Smallpox was indeed endemic in many developing countries in the 1960s, causing significant morbidity and mortality. This is supported by historical records and epidemiological data from that time period. Choices B, C, and D are incorrect because smallpox was not endemic in North America, England, or globally during the 1960s. Smallpox eradication efforts led to the global eradication of the disease in 1980, but in the 1960s, it was still prevalent in many developing countries.

Correct Answer: A

Rationale: The correct answer is A: CD4+ T cells. HIV primarily infects CD4+ T cells by binding to the CD4 receptor and a co-receptor, usually CCR5 or CXCR4. CD4+ T cells are crucial for coordinating the immune response. B cells (choice B) are not typically infected by HIV. CD8+ T cells (choice C) can be indirectly affected by HIV, but they are not the primary target of the virus. Therefore, the correct choice is A as it accurately reflects the primary target cells of HIV infection.

Correct Answer: B

Rationale: Correct Answer: B (VDJ gene rearrangement) Rationale: 1. VDJ gene rearrangement occurs during B cell development in the bone marrow, independent of T helper cells. 2. Somatic hypermutation and immunoglobulin class switching require T helper cell interactions. 3. Memory cell differentiation also involves T helper cells for activation and differentiation. 4. Therefore, VDJ gene rearrangement is the only process not directly dependent on antigen-specific T helper cells.

Correct Answer: A

Rationale: Neutrophils are typically not found in the lamina propria of the gut in the absence of infection because they are primarily involved in the acute inflammatory response to infections. Macrophages, plasma cells, and T cells are commonly present in the gut lamina propria to maintain immune homeostasis and respond to antigens. Neutrophils are more abundant in tissues during active infection due to their role in phagocytosis and inflammation. Therefore, the correct answer is A: Neutrophils.

Correct Answer: D

Rationale: The correct answer is D. By engineering a form of the antibody with human variable region protein sequences (choice B), it reduces the risk of immune response in patients. Similarly, by engineering a form with human constant region protein sequences (choice C), it further reduces immunogenicity. Therefore, both B and C contribute to making the antibody more suitable for human use, enhancing its efficacy and safety in treating neuroblastoma. Choices A and D are incorrect as they do not address the crucial aspect of reducing immunogenicity through humanization of the antibody.