The discovery of immunoglobulin gene rearrangement helped to explain:

Correct Answer: A

Rationale: The correct answer is A: How the number of different immunoglobulins produced by an individual can be far greater than the number of genes in the human genome. Immunoglobulin gene rearrangement occurs during B cell development, leading to the generation of a diverse repertoire of antibodies from a limited number of genes. This process allows for the production of a vast array of immunoglobulins through mechanisms like V(D)J recombination. Choice B is incorrect because the gene rearrangement process actually enables a single gene to encode for multiple immunoglobulins. Choice C is partially correct but does not fully capture the significance of gene rearrangement in expanding the diversity of immunoglobulins beyond what the genome would suggest. Choice D is incorrect as it combines a correct statement (A) with an incomplete statement (C).

Correct Answer: D

Rationale: The correct answer is D because all the factors mentioned are important in the eradication of smallpox. A is correct because smallpox is specific to humans, simplifying eradication efforts. B is correct as the vaccine was easily deployable. C is correct as isolation of symptomatic individuals helped control the spread. Therefore, all choices are important in the successful eradication of smallpox.

Correct Answer: B

Rationale: The correct answer is B because innate immune cells do not respond better and faster at a second encounter with the same pathogen. This is a characteristic of adaptive immunity, not innate immunity. During a second encounter, adaptive immune cells produce a faster and stronger response due to memory cells formed during the initial exposure. In contrast, innate immune cells do not have memory cells and their response remains the same upon repeated encounters. Choice A is correct as innate cells are immediately active and do not require clonal amplification or differentiation. Choice C is correct as innate receptors recognize broad structures on pathogens. Choice D is correct as innate receptors can recognize modified-self, such as in autoimmune diseases.

Correct Answer: D

Rationale: Neutrophils lack the ability to present antigens to T cells due to their short lifespan and absence of major histocompatibility complex (MHC) class II molecules. This process requires MHC class II presentation, which is typically done by antigen-presenting cells such as dendritic cells, macrophages, and B cells. Neutrophils primarily function in phagocytosis, releasing reactive oxygen species (ROS), defensins, and neutrophil extracellular traps (NETs) to combat pathogens. Thus, the correct answer is D. Other choices are incorrect because neutrophils are capable of producing ROS (A), defensins (B), and NETs (C).

Correct Answer: B

Rationale: Rationale for Answer B: 1. Antibody structure consists of two identical heavy chains and two identical light chains. 2. These chains are covalently linked to form the Y-shaped structure of an antibody. 3. Each chain contributes to the formation of antigen-binding sites. 4. Therefore, two identical heavy chains and two identical light chains form two antigen-binding sites. Summary of Incorrect Choices: A: Incorrect because it describes only one antigen-binding site formed by one light chain and one heavy chain. C: Incorrect because it describes one antigen-binding site formed by two identical heavy chains and two identical light chains. D: Incorrect because it incorrectly describes the roles of light and heavy chains in antibody structure.