After binding to its specific antigen, a B lymphocyte may switch its

Correct Answer: B

Rationale: The correct answer is B: immunoglobulin heavy-chain class. After binding to its specific antigen, a B lymphocyte can undergo class switching to change the type of antibody it produces while maintaining the same antigen specificity. This process involves recombination events that switch the constant region of the immunoglobulin heavy chain, leading to the production of antibodies with different effector functions. Choices A, C, and D are incorrect because they do not accurately describe the process of class switching in B lymphocytes. Light-chain isotypes, variable region of the heavy chain, and constant region of the light chain do not undergo class switching.

Correct Answer: D

Rationale: The correct answer is D because toxic shock syndrome (TSS) is caused by superantigens, not A-B toxins. Superantigens directly activate a large number of T cells by binding to both the MHC class II protein and the T cell receptor simultaneously. This leads to massive cytokine release and the symptoms described. Choices A, B, and C are all accurate statements about the mechanism of action of superantigens in TSS. Choice D is incorrect because TSS toxins do not have an A-B subunit structure; they are superantigens that directly interact with T cells and do not enter cells to activate them.

Correct Answer: D

Rationale: The correct answer is D: enzymes released by polymorphonuclear cells. When immune complexes are deposited in the glomerular basement membrane, neutrophils (a type of polymorphonuclear cell) are recruited. These neutrophils release enzymes such as elastase and collagenase, which directly damage the membrane. Gamma interferon (choice A) is involved in immune response but does not directly damage the membrane. Phagocytosis (choice B) is the process of engulfing and digesting particles, not directly causing damage to the membrane. Cytotoxic T cells (choice C) target and kill infected or abnormal cells, but they are not directly involved in damaging the glomerular basement membrane in this scenario.

Correct Answer: C

Rationale: The correct answer is C because complement lyses cells by forming a membrane attack complex (MAC) which involves the insertion of complement proteins into the cell membrane, leading to cell lysis. Choices A, B, and D are incorrect as complement does not lyse cells through enzymatic digestion, activation of adenylate cyclase, or inhibition of elongation factor 2. Complement-mediated lysis specifically involves the formation of MAC to disrupt the target cell membrane.

Correct Answer: D

Rationale: The correct answer is D because in autoimmune hemolytic anemia, activation of the complement system leads to the formation of membrane attack complexes (MAC) that directly lyse the red blood cells. This process is known as complement-mediated hemolysis. Perforins from cytotoxic T cells (Choice A) are involved in cell-mediated cytotoxicity but are not typically responsible for hemolysis of red blood cells. Neutrophils releasing proteases (Choice B) or interleukin-2 binding to its receptor on red cells (Choice C) are not mechanisms commonly associated with hemolysis in autoimmune hemolytic anemia.