All of the following are true about antibodies, EXCEPT which one?

Correct Answer: D

Rationale: The correct answer is D because antibodies do not have a single, defined amino acid sequence due to variability in their structure. This variability is a result of somatic recombination and hypermutation. A: Antibodies can fix complement, leading to the destruction of pathogens. B: Antibodies are produced by B-lymphocytes and can occur on their surface as B-cell receptors. C: Antibodies predominate the secondary immune response, not the primary response, as memory B-cells are generated during the primary response for a quicker and more robust secondary response.

Correct Answer: C

Rationale: The correct answer is C: Carefully splint the leg as it is. This is the most appropriate immediate action because splinting the leg as it is will help stabilize the fracture and prevent further injury or damage to surrounding tissues. Moving or straightening the leg (option D) could worsen the fracture and cause more pain. Positioning him flat on his back (option A) may not provide enough support and could lead to additional complications. Applying a tourniquet (option B) is not necessary and could potentially cut off blood flow to the leg, causing more harm. Splinting the leg as it is will help maintain alignment and reduce the risk of further damage until proper medical care can be provided.

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.