What lineage do natural killer (NK) cells belong to?

Correct Answer: C

Rationale: The correct answer is C: Lymphoid lineage. NK cells are a type of lymphocyte that is part of the innate immune system. They are derived from common lymphoid progenitors in the bone marrow. NK cells play a crucial role in recognizing and killing infected or abnormal cells. Myeloid lineage (A) gives rise to cells like macrophages and granulocytes, not NK cells. Erythroid lineage (B) produces red blood cells, and granulocyte lineage (D) gives rise to neutrophils, basophils, and eosinophils, not NK cells. Therefore, the correct choice is C as NK cells originate from the lymphoid lineage.

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.

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.