The rearrangement of gene segments that occurs randomly in B cell receptor genes and T cell receptor genes:

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: 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.

Correct Answer: C

Rationale: Step-by-step rationale: 1. Hybridoma is a cell line formed by the fusion of a B cell (producing a specific antibody) and a myeloma cell. 2. Choice C is correct because hybridomas can be immortalized and cultured indefinitely. 3. Choice A is incorrect because hybridomas produce monoclonal antibodies, not polyclonal antibodies. 4. Choice B is incorrect because hybridomas are formed from a single B cell and a myeloma cell, not two antigen-activated B cells. 5. Choice D is incorrect as only option C accurately describes a hybridoma's characteristic of indefinite tissue culture survival.