Which complement pathway is triggered by mannose-binding lectin (MBL)?

Correct Answer: C

Rationale: The correct answer is C, Lectin pathway. MBL recognizes pathogen-associated molecular patterns containing mannose, initiating the lectin pathway. MBL binds to mannose on microbial surfaces, activating MBL-associated serine proteases, leading to the formation of the C3 convertase. This triggers the rest of the complement cascade. A: Alternative pathway is triggered by spontaneous hydrolysis of C3, not by MBL. B: Classical pathway is activated by antigen-antibody complexes, not MBL. D: Terminal pathway (also known as the membrane attack complex) is the final step of all complement pathways, not specifically triggered by MBL.

Correct Answer: B

Rationale: The correct answer is B: TLR4. TLR4 is primarily responsible for recognizing lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria. It forms a complex with MD-2 and CD14 to trigger downstream signaling pathways. TLR2 recognizes other microbial components such as lipoproteins. TLR5 recognizes flagellin, a component of bacterial flagella. TLR9 recognizes unmethylated CpG DNA motifs. Therefore, TLR4 is specifically involved in detecting LPS, making it the correct answer in this scenario.

Correct Answer: B

Rationale: The correct answer is B: Clonal selection. During clonal selection, a specific lymphocyte recognizes its specific antigen, binds to it, and undergoes proliferation to generate a large population of identical cells to combat the antigen efficiently. This process ensures an effective immune response. Incorrect choices: A: Clonal deletion - This process involves the elimination of self-reactive lymphocytes to prevent autoimmunity, not the proliferation of antigen-specific lymphocytes. C: Antigen presentation - This is the process by which antigens are displayed to immune cells for recognition, not the proliferation of lymphocytes. D: Cytokine signaling - While cytokines play a role in immune responses, they are not directly involved in the proliferation of lymphocytes in response to specific antigens.

Correct Answer: B

Rationale: The correct answer is B: IgG. IgG is the most abundant antibody isotype in circulation, constituting about 75% of total antibodies. This is because IgG is produced during secondary immune responses and has a longer half-life compared to other isotypes, allowing it to accumulate in circulation. IgA is mainly found in mucosal surfaces, IgE is associated with allergic reactions, and IgM is the first antibody produced in a primary immune response. Therefore, IgG is the most abundant in circulation due to its longevity and role in secondary immune responses.

Correct Answer: A

Rationale: The correct answer is A: Bone marrow. B cells mature in the bone marrow where they develop from hematopoietic stem cells. This process involves gene rearrangement and selection for self-tolerance. The bone marrow provides a microenvironment necessary for B cell development. Choices B, C, and D are incorrect because the thymus is where T cells mature, the spleen is involved in immune responses but not B cell maturation, and lymph nodes are sites for immune cell activation and proliferation but not B cell maturation.