Which category of hypersensitivity BEST describes hemolytic disease of the newborn caused by Rh incompatibility?

Correct Answer: D

Rationale: The correct answer is D: delayed hypersensitivity. Hemolytic disease of the newborn caused by Rh incompatibility is a type II hypersensitivity reaction. Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus, leading to the mother producing antibodies against the Rh antigen. This immune response is delayed and involves sensitized T cells. This results in the destruction of Rh-positive red blood cells in the newborn. Choice A (atopic or anaphylactic) refers to type I hypersensitivity, which involves immediate IgE-mediated reactions like allergies. Choice B (cytotoxic) is a close contender, as it also involves antibodies targeting specific antigens on cell surfaces, but it typically involves antibody-dependent cell-mediated cytotoxicity rather than delayed responses. Choice C (immune complex) relates to type III hypersensitivity, where immune complexes deposit in tissues and activate complement, causing inflammation. However, this mechanism is not the primary process in hemolytic disease of the newborn caused by

Correct Answer: B

Rationale: The correct answer is B: increased numbers of severe bacterial infections. Severely reduced C3 levels indicate a deficiency in the complement system, which plays a key role in immune defense against bacteria. Without C3, the ability to opsonize bacteria and enhance phagocytosis is impaired, leading to an increased susceptibility to severe bacterial infections. The other choices are incorrect because low gamma globulin levels are associated with antibody deficiencies, severe viral infections are typically more common with T-cell deficiencies, and hemolytic anemia is linked to issues with red blood cell destruction rather than bacterial infections.

Correct Answer: A

Rationale: The correct answer is A: Mu chain. During B lymphocyte maturation, the first immunoglobulin heavy chain synthesized is the Mu chain. This is because the Mu chain is the first heavy chain isotype expressed during early B cell development. The Mu chain combines with the surrogate light chain to form the pre-B cell receptor, which is essential for signaling in pre-B cells. The Mu chain is later replaced by other heavy chain isotypes such as gamma, epsilon, and alpha during B cell differentiation. Therefore, the Mu chain is the correct answer as it is the initial heavy chain produced in B cell maturation. Choices B, C, and D are incorrect because they represent heavy chain isotypes that are expressed at later stages of B cell development and differentiation.

Correct Answer: D

Rationale: The correct answer is D because minor histocompatibility antigens can induce immune reactions leading to rejection in transplantation. These antigens are derived from genetic variations, not detected by antibodies and complement (A), controlled by genes outside the major histocompatibility complex (B), and are important in transplantation as they contribute to rejection responses (C).

Correct Answer: D

Rationale: The correct answer is D: class II MHC antigens. Antigen-presenting cells activate helper T cells by presenting antigens on their surfaces via class II MHC molecules. These molecules bind to the T cell receptor, triggering the immune response. IgE (choice A) is not involved in activating helper T cells. Gamma interferon (choice B) is a cytokine produced by T cells but is not directly involved in antigen presentation. Class I MHC antigens (choice C) present antigens to cytotoxic T cells, not helper T cells.