Which of the following early contributors to the development of immunology is paired up properly with his or her accomplishment?

Correct Answer: D

Rationale: Rationale: Robert Koch is correctly paired with developing techniques to grow pure bacterial cultures. Koch's work laid the foundation for bacteriology and allowed for the isolation and identification of specific disease-causing bacteria. This technique, known as Koch's postulates, revolutionized the field of microbiology by linking specific pathogens to specific diseases. Summary: A: Louis Pasteur - Incorrect. While Pasteur is known for his work in vaccination and pasteurization, the provided accomplishment is associated with Edward Jenner, not Pasteur. B: Paul Ehrlich - Incorrect. Ehrlich is known for his work on immunology and coined the term "magic bullet," but he did not identify lymphocytes as cells mediating adaptive immunity. C: MacFarlane Burnet - Incorrect. Burnet is known for his clonal selection theory, not the side-chain theory of antibodies as described in the choice.

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: Step 1: Gene rearrangement in B cell and T cell receptor genes creates unique receptor proteins. Step 2: This diversity allows mature lymphocytes to recognize a wide range of antigens. Step 3: This process is crucial for adaptive immune response and antigen specificity. Step 4: Choices A and B do not accurately describe gene rearrangement. Step 5: Choice D is incorrect as gene rearrangement is specific to lymphocytes, not all cells in the body.

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.