Iron is incorporated into the heme molecule in which of the following forms:

Correct Answer: B

Rationale: The correct answer is B: Ferrous. In the heme molecule, iron is present in the ferrous form (Fe2+). This form of iron allows for the binding of oxygen in hemoglobin. Ferro (A) is not a correct term for iron in heme. Ferric (C) refers to the Fe3+ form of iron, which is not found in heme. Apoferritin (D) is a protein that stores iron but is not directly involved in heme synthesis. Therefore, the correct form of iron in the heme molecule is ferrous (B).

Correct Answer: B

Rationale: The correct answer is B because the principal difference between cytotoxic (type II) and immune complex (type III) hypersensitivity lies in the site where antigen-antibody complexes are formed. In type II hypersensitivity, the complexes form on the surface of target cells, leading to cell destruction. In contrast, in type III hypersensitivity, the complexes form in circulation or in tissues, leading to immune complex deposition and subsequent tissue damage. The other choices are incorrect because the class of antibody (A), participation of complement (C), and participation of T cells (D) can be common features in both type II and type III hypersensitivity reactions, but the crucial distinguishing factor is where the complexes are formed.

Correct Answer: C

Rationale: The correct answer is C because AIDS is caused by the human immunodeficiency virus (HIV), which specifically targets and kills CD4-positive T lymphocytes. These cells are crucial for coordinating the immune response. A, B, and D are incorrect because HIV primarily affects CD4-positive T cells, not B lymphocytes, lymphocyte stem cells, or CD8-positive T cells. This leads to a weakened immune system, making individuals more susceptible to infections and other complications associated with AIDS.

Correct Answer: D

Rationale: The correct answer is D: decreased resistance to Neisseria bacteremia. This is because C6 is a component of the complement system, which plays a crucial role in the immune response against Neisseria bacteria. A deficiency in C6 would impair the ability to effectively combat Neisseria infections. Choice A is incorrect because the deficiency of C6 does not directly affect resistance to viral infections. Choice B is incorrect as hypersensitivity reactions are not specifically linked to C6 deficiency. Choice C is incorrect because there is no direct correlation between C6 deficiency and an increased frequency of cancer.

Correct Answer: B

Rationale: The correct answer is B because helper T cells play a crucial role in activating B cells to produce antibodies. In this scenario, the protein component of the hapten-protein conjugate must be recognized by helper T cells to initiate the immune response. This recognition leads to the activation and differentiation of B cells into plasma cells that secrete anti-hapten antibodies. Choices A, C, and D are incorrect because the focus should be on the protein component, not the hapten or suppressor T cells, for the production of anti-hapten antibodies.