How do NK cells kill target cells?

Correct Answer: C

Rationale: The correct answer is C. NK cells kill target cells by releasing perforin and granzyme. Perforin creates pores in the target cell's membrane, allowing granzyme to enter and induce apoptosis. Antibodies (choice A) are produced by B cells, not NK cells. Cytokines (choice B) regulate immune responses but do not directly kill cells. Phagocytosis (choice D) involves engulfing and digesting pathogens, which is not how NK cells kill target cells.

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: C

Rationale: The correct answer is C: Streptococcus pneumoniae infections. Complement deficiencies impair the body's ability to fight bacterial infections, particularly encapsulated bacteria like Streptococcus pneumoniae. Asthma (A), SLE (B), and chronic hepatitis (D) are not directly associated with increased susceptibility to bacterial infections due to complement deficiencies. Asthma is a chronic respiratory condition, SLE is an autoimmune disease, and chronic hepatitis affects the liver. Therefore, only Streptococcus pneumoniae infections are directly linked to complement deficiencies.

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.