A major effector function of TH2 cells is:

Correct Answer: B

Rationale: The correct answer is B because TH2 cells play a crucial role in protecting against parasitic worms by secreting cytokines that promote eosinophil activation and antibody production. This response helps to expel the parasites from the body. Incorrect choices: A: TH1 cells are responsible for protection against intracellular bacteria. C: TH3 cells, not TH2 cells, are known for secreting anti-inflammatory cytokines. D: Activation of macrophages is primarily mediated by TH1 cells through IFN-gamma secretion.

Correct Answer: D

Rationale: Pattern recognition receptors (PRRs) are a class of receptors that recognize specific molecular patterns associated with pathogens. They can be found in various locations within the body, including in soluble form in the blood (Choice A), in the cytosol of cells (Choice B), and on endosomal membranes (Choice C). Therefore, the correct answer is D, "All of the above are correct," as PRRs can indeed be located in all of these locations. PRRs play a crucial role in the innate immune response by detecting and responding to pathogens, making their presence in multiple cellular compartments essential for effective immune surveillance and defense.

Correct Answer: D

Rationale: The correct answer is D, "All of the above." Mast cells, eosinophils, and basophils are all involved in allergic reactions. Mast cells release histamine, leading to inflammation. Eosinophils are involved in allergic responses and parasitic infections. Basophils release histamine and other inflammatory mediators. Therefore, all three cell types play a crucial role in allergic reactions. Choices A, B, and C alone are incorrect because each cell type individually participates in allergic reactions.

Correct Answer: A

Rationale: The correct answer is A: Light chains isotypes are functionally different. This statement is NOT true because light chain isotypes (kappa and lambda) have distinct primary structures but serve the same function in antigen binding. The heavy chain determines the effector function of antibodies. The other choices are correct: B) The antigen binding site of a B cell receptor is identical to the antigen binding site of antibodies secreted by the same B cell, as both use the same variable regions for antigen recognition. C) Both heavy and light chain domains assume a compact barrel structure, which is essential for antibody stability and function. D) The hinge region assumes a more open and flexible structure, allowing antibodies to bind antigens at different orientations.

Correct Answer: C

Rationale: The correct answer is C because a closed circulatory system means that blood stays confined to vessels, providing efficient transport and preventing blood loss. Option A is incorrect because a closed system does not have open vessels. Option B is incorrect because veins and arteries are part of both closed and open circulatory systems. Option D is incorrect because returning blood to the heart is a function of both closed and open circulatory systems.