During an adaptive immune response to a pathogen an important outcome of the clonal selection process is:

Correct Answer: A

Rationale: The correct answer is A because during clonal selection, lymphocytes specific to the pathogen are activated, leading to their proliferation and differentiation into effector cells. This results in an increase in the number of lymphocytes specific for the pathogen, enhancing the immune response. Choice B is incorrect because clonal selection does not involve the elimination of non-specific lymphocytes but rather the expansion of specific lymphocytes. Choice C is also incorrect because clonal selection does not change the antigen receptor specificities expressed by lymphocytes but instead focuses on expanding the population of lymphocytes with receptors specific to the pathogen. Therefore, the correct outcome of clonal selection is the increase in the number of lymphocytes specific for the pathogen that activated the response, making choice A the most accurate option.

Correct Answer: A

Rationale: The correct answer is A because cytotoxic T lymphocytes (CTLs) kill target cells by recognizing specific peptide antigens presented on major histocompatibility complex (MHC) class I molecules. This recognition triggers the CTLs to release cytotoxic molecules that induce apoptosis in the target cells. Choice B is incorrect because CTLs do not die in the process of killing target cells; they survive to continue their immune response. Choice C is incorrect as CTLs do not kill target cells by activating complement, but rather through direct cell-cell contact. Choice D is incorrect because CTLs carry out their effector function in peripheral tissues where they encounter target cells, not specifically in secondary lymphoid tissues.

Correct Answer: B

Rationale: Positive selection is necessary to produce a T-cell repertoire capable of interacting with self MHC molecules. During positive selection in the thymus, T-cells that can recognize self MHC molecules are retained, ensuring self-tolerance and functionality. Negative selection eliminates self-reactive T-cells. Induction of anergy is a state of T-cell unresponsiveness and does not contribute to the development of a functional T-cell repertoire. Choice A is incorrect as only positive selection is necessary for self MHC recognition.

Correct Answer: C

Rationale: The correct answer is C: 1-b, 2-d, 3-a, 4-c. To determine this, we need to match each item correctly. Step by step: 1) First item is matched with b because it corresponds to the correct sequence. 2) Second item is matched with d as it follows the order. 3) Third item is matched with a as it aligns with the pattern. 4) Fourth item is matched with c to complete the sequence. Other choices lack logical consistency in matching the items as per their correct order, making them incorrect.

Correct Answer: D

Rationale: The correct answer is D: Immunoglobulin A. Colostrum contains high levels of Immunoglobulin A (IgA), which is crucial for imparting passive immunity to the newborn. IgA helps protect the infant's mucosal surfaces from infections. Natural killer cells, monocytes, and macrophages are not typically found in colostrum and are not responsible for conferring immunity to the newborn.