The class of an immunoglobulin

Correct Answer: D

Rationale: The correct answer is D because the class of an immunoglobulin is determined by the type of heavy chain it possesses. There are five main classes of immunoglobulins (IgG, IgA, IgM, IgD, IgE) which are defined by the heavy chain constant region. Different heavy chain types result in different effector functions and biological properties. Choices A, B, and C are incorrect because the class of an immunoglobulin is not determined by Class I and Class II major histocompatibility complex proteins, carbohydrate attached to the light chain, or the antigen. These factors may influence the function or specificity of the antibody, but they do not determine its class.

Correct Answer: B

Rationale: The correct answer is B because IgG, not IgM, is the best at facilitating phagocytosis through the Fc receptor. IgM is the best at activating complement (A), antibodies can neutralize bacterial toxins (C), and antibodies can inhibit binding of toxins to their receptors on host cells (D). IgM primarily activates complement, while IgG is more efficient at facilitating phagocytosis through the Fc receptor. This distinction is crucial in understanding the functions of different immunoglobulins.

Correct Answer: D

Rationale: The correct answer is D, TH1 differentiation vs TH2 differentiation, as it is not a random event. TH1 and TH2 differentiation is determined by cytokine signals, not random chance. A, Genetic recombination of VDJ segments for TCR assembly, and B, Genetic recombination of VDJ segments for BCR assembly, involve random processes during immune cell development. C, CD4 vs CD8 T cell fate, is determined by random rearrangement of gene segments during T cell development. Thus, D is the only choice that does not involve a random event in immune cell development.

Correct Answer: A

Rationale: The correct answer is A because developing T cells initially express both CD4 and CD8 co-receptors in the thymus. This stage is crucial for T cell maturation and selection. Eventually, T cells undergo positive and negative selection processes to become either CD4+ helper T cells or CD8+ cytotoxic T cells based on their specificity and function. Choices B and C are incorrect because T cells initially express both CD4 and CD8. Choice D is also incorrect as T cells do express either CD4 or CD8 during their development in the thymus.

Correct Answer: D

Rationale: Step 1: HIV is a retrovirus because it contains RNA as its genetic material and uses the enzyme reverse transcriptase to convert its RNA into DNA. Step 2: HIV is an RNA virus because its genetic material is RNA. Step 3: HIV is a lentivirus which is a subgroup of retroviruses known for causing slow-progressing diseases. Therefore, the correct answer is D as all statements A, B, and C are true based on the characteristics of HIV.