Which cellular structure is involved in digesting engulfed pathogens during phagocytosis?

Correct Answer: A

Rationale: The correct answer is A: Lysosomes. Lysosomes contain enzymes that break down engulfed pathogens during phagocytosis. First, the pathogen is engulfed by the cell membrane forming a phagosome. Next, the phagosome fuses with a lysosome forming a phagolysosome where digestion occurs. Golgi apparatus (B) is involved in protein processing and packaging. Endoplasmic reticulum (C) is responsible for protein synthesis and lipid metabolism. Mitochondria (D) produce energy in the form of ATP.

Correct Answer: B

Rationale: The correct answer is B: Macfarlane Burnet. Burnet proposed the concept of clonal selection in the 1950s to explain how the immune system recognizes and responds to specific antigens. He suggested that lymphocytes with receptors specific to antigens undergo clonal expansion upon encountering the antigen. This hypothesis laid the groundwork for our understanding of adaptive immunity. A: Paul Ehrlich developed the side-chain theory of antibody formation, not clonal selection. C: Louis Pasteur is known for his work in microbiology and vaccination, not clonal selection. D: Elie Metchnikoff is credited with discovering phagocytosis, not clonal selection.

Correct Answer: D

Rationale: The correct answer is D: IgE. IgE is the antibody isotype involved in allergic reactions. It binds to allergens and triggers the release of histamine and other chemicals, leading to allergic symptoms. IgG (choice A) is involved in long-term immunity, IgM (choice B) is the first antibody produced in response to an infection, and IgA (choice C) is mainly found in mucosal areas and helps prevent pathogens from entering the body. IgE is specifically associated with allergic responses due to its role in triggering the release of inflammatory mediators in response to allergens.

Correct Answer: B

Rationale: The correct answer is B because lymphocytes are activated in adaptive immunity by recognizing their specific antigen. This recognition is crucial for initiating an immune response tailored to combat a specific pathogen. Phagocytosis (A) is the process of engulfing and destroying pathogens, but it does not directly activate lymphocytes. Cytokine release by macrophages (C) can help in activating other immune cells, but lymphocytes specifically require antigen recognition for activation. Opsonization of pathogens (D) enhances phagocytosis but is not the direct trigger for lymphocyte activation.

Correct Answer: C

Rationale: The correct answer is C: IgG. In the secondary immune response, IgG antibodies predominate due to memory B cells rapidly producing them upon re-exposure to the same antigen. IgG provides long-lasting immunity and is more efficient in neutralizing pathogens. IgM (A) is the primary antibody in the initial immune response. IgE (B) is involved in allergic reactions. IgD (D) functions as a receptor on B cells.