The differentiation of activated Th cells into various effector subsets depends mostly on:

Correct Answer: B

Rationale: The correct answer is B because polarizing cytokines secreted by dendritic cells play a crucial role in determining the differentiation of activated Th cells into specific effector subsets. These cytokines guide the Th cells towards different functional phenotypes, such as Th1, Th2, Th17, or Treg cells. This process helps tailor the immune response to the specific type of pathogen encountered. Choices A, C, and D are incorrect because while signal 1 of activation, T cell receptor co-receptors, and duration of signaling are important factors in T cell activation and differentiation, they do not have as direct and significant an impact on effector subset differentiation as the influence of polarizing cytokines.

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.

Correct Answer: C

Rationale: The correct answer is C: Ethylene. Ethylene is indeed a gaseous plant hormone produced during the combustion of natural gas. It plays a crucial role in various plant processes such as fruit ripening, senescence, and stress responses. A: Auxin is not a gaseous hormone and is primarily involved in promoting cell elongation and root growth. B: Gibberellins are not gaseous hormones and mainly regulate plant growth processes like stem elongation and seed germination. D: The statement "All above" is incorrect as auxin and gibberellins are not gaseous hormones, only ethylene fits the description. Therefore, choice C, Ethylene, is the correct answer as it is the gaseous by-product of natural gas combustion and a plant hormone.