Which of the following molecules can function both as complement initiators and as opsonins?

Correct Answer: D

Rationale: The correct answer is D, All of the above. C3b, C-reactive protein, and MBL can all function as complement initiators and opsonins. C3b is a key complement protein involved in both processes. C-reactive protein can activate complement and enhance phagocytosis. MBL can also activate complement and enhance phagocytosis by binding to pathogens. All choices can serve as both initiators of the complement cascade and as opsonins, making D the correct answer.

Correct Answer: C

Rationale: Rationale: 1. Tumor antigens are non-self antigens that trigger a weak immune response due to immune tolerance. 2. Weak immune response against tumor antigens may lead to uncontrolled cell growth, resulting in cancer. 3. Strong immune responses are typically mounted against pathogens to clear infections. 4. Self-antigens should not trigger immune responses to prevent autoimmune diseases. 5. Transplanted organs can elicit strong immune responses leading to rejection, not acceptance. Summary: A - Incorrect: Self-antigens should not elicit immune responses to prevent autoimmune diseases. B - Incorrect: Pathogens typically trigger strong immune responses to clear infections, not recurrent ones. D - Incorrect: Transplanted organs often lead to strong immune responses and rejection, not acceptance.

Correct Answer: B

Rationale: The correct answer is B: CTLA-4. CTLA-4 inhibits T cell activation by binding to CD80/CD86 on antigen-presenting cells, preventing the co-stimulatory signal required for T cell activation. It is highly expressed in Tregs, which suppress immune responses. Recombinant CTLA-4 (e.g., abatacept) is used to treat autoimmune diseases by blocking T cell activation. Explanation for other choices: A: CD86 is a co-stimulatory molecule that activates T cells, not inhibits them. C: MHC class II molecules present antigens to T cells, they do not inhibit T cell activation. D: CD4 is a co-receptor that helps T cells recognize antigens presented by MHC class II molecules, it does not inhibit T cell activation.

Correct Answer: B

Rationale: Rationale: 1. Affinity maturation is the process where B cells produce antibodies with increased affinity for the antigen over time. 2. B cells whose antibodies bind the antigen more strongly have higher affinity. 3. Survival of B cells with higher affinity antibodies is crucial for effective immune response. 4. A is incorrect as rate of antibody secretion doesn't necessarily correlate with affinity. 5. C is incorrect as T cells are not directly involved in affinity maturation of antibodies. 6. D is incorrect since T cells are not affected by affinity maturation in the context of this question.

Correct Answer: D

Rationale: The correct answer is D because monoclonal antibodies have the ability to block the proliferation of cancer cells by targeting specific antigens on the cancer cells. They can also target cancer cells for destruction through various mechanisms such as antibody-dependent cell-mediated cytotoxicity. Additionally, monoclonal antibodies can inhibit inflammatory immune responses by neutralizing pro-inflammatory mediators or blocking immune cell activation pathways. Therefore, all the options A, B, and C are correct, making D the correct answer.