The 'major histocompatibility complex' (MHC) proteins involved in antigen presentation to T cells were first noted during studies of:

Correct Answer: C

Rationale: The correct answer is C: Transplantation. MHC proteins play a crucial role in transplant rejection by presenting antigens to T cells, initiating an immune response against foreign tissues. This discovery was significant in understanding transplant immunology. Choices A, B, and D are incorrect because innate immunity involves nonspecific defense mechanisms, allergies are mediated by IgE antibodies, and autoimmunity involves the immune system attacking self-antigens, none of which directly relate to the discovery of MHC proteins in transplantation studies.

Correct Answer: B

Rationale: The correct answer is B because lysozyme is not a type of defensin. Lysozyme is an enzyme present in secretions and tears, part of chemical innate barriers to infection, and capable of dissolving the cell wall of bacteria. Defensins are a different type of antimicrobial peptides that function by disrupting the microbial cell membrane. Therefore, B is the correct answer as it does not accurately describe lysozyme's function.

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.