Which of the following is correct with respect to measles:

Correct Answer: A

Rationale: Rationale for Choice A: Outbreaks of measles in developed countries are due to decreased vaccination rates, leading to lower herd immunity. This is supported by data showing increasing cases in countries with vaccine hesitancy. Other choices are incorrect because B is false - measles has not been eradicated globally, C is false - there is an effective vaccine for measles, and D is false - measles can lead to severe complications and long-term effects, especially in unvaccinated individuals.

Correct Answer: B

Rationale: The correct answer is B because the major histocompatibility complex (MHC) plays a crucial role in the immune system by determining whether transplanted organs or cells will be accepted or rejected. MHC molecules present antigens to T cells, which then trigger an immune response leading to acceptance or rejection. Choice A is incorrect because T cell receptor gene rearrangement is not directly influenced by the MHC. Choice C is incorrect as MHC does not directly polarize T helper cells. Choice D is incorrect because MHC does not directly regulate the ratio of CD4+ and CD8+ T cells in an individual.

Correct Answer: C

Rationale: Passive immunization involves the transfer of pre-formed antibodies to confer immediate protection. Choice C is correct as it describes receiving serum containing antibodies from an immune individual. Choice A is incorrect because it describes active immunization. Choice B is incorrect as it refers to the primary immune response, which is part of active immunization. Choice D is incorrect as it describes the process of immunization itself, not passive immunization.

Correct Answer: A

Rationale: The correct answer is A: 1-C, 2-A, 3-B, 4-D. Immunoglobulins are antibodies with different functions: 1. IgG (C) - Main antibody in secondary immune response. 2. IgA (A) - Found in mucosal areas, neutralizes pathogens. 3. IgM (B) - First antibody produced in primary immune response. 4. IgE (D) - Involved in allergic reactions and defense against parasites. Other choices are incorrect as they mismatch the immunoglobulins with their functions.

Correct Answer: B

Rationale: The correct answer is B: Typhoid. Live attenuated vaccines contain weakened forms of the pathogen that can replicate in the host to stimulate a strong immune response without causing disease. Typhoid vaccine is an example of a live attenuated bacterial vaccine because it contains weakened Salmonella typhi bacteria. Cholera (A), Anthrax (C), and Plague (D) vaccines are not live attenuated bacterial vaccines; they are either killed/inactivated or subunit vaccines. Live attenuated vaccines provide longer-lasting immunity compared to other vaccine types.