Phagocytes were discovered by:

Correct Answer: A

Rationale: The correct answer is A: Ilya Metchnikoff. He discovered phagocytes through his research on immune responses in starfish larvae. Metchnikoff observed cells engulfing foreign particles, leading to the discovery of phagocytosis. Louis Pasteur is famous for his work in microbiology and vaccination. Emil von Behring discovered antitoxins. Robert Koch is known for his work in bacteriology and identifying the causative agents of diseases. Therefore, the correct answer is A as Metchnikoff specifically discovered phagocytes.

Correct Answer: C

Rationale: The lectin and classical pathways both lead to the formation of the same C3 convertase (C4b2a). This is because in the lectin pathway, mannose-binding lectin (MBL) binds to specific sugar residues on pathogens, activating MBL-associated serine proteases (MASPs) that cleave C4 and C2 to form the C3 convertase. Similarly, in the classical pathway, C1 complex binds to antigen-antibody complexes, activating C1s to cleave C4 and C2 to form the C3 convertase. This convergent step in both pathways results in the cleavage of C3 into C3a and C3b, leading to downstream complement activation. The other choices are incorrect because they do not accurately reflect the key similarity between the lectin and classical pathways in terms of the formation of the C3 convertase.

Correct Answer: D

Rationale: The correct answer is D because the secondary immune response is faster and stronger compared to the primary response. This is due to the presence of memory B and T cells from the primary response that can quickly recognize and respond to the antigen upon re-exposure. These memory cells allow for a more rapid and robust immune response, leading to quicker elimination of the pathogen. Choices A and B are incorrect because both primary and secondary responses involve the adaptive immune system and are antigen-specific. Choice C is incorrect as the secondary response is faster and more potent than the primary response.

Correct Answer: C

Rationale: The rearrangement of B cell antigen receptor genes occurs in the bone marrow. This process involves the assembly of variable (V), diversity (D), and joining (J) gene segments to generate a diverse repertoire of B cell receptors. The bone marrow provides the necessary environment for this gene rearrangement to take place, ensuring the development of functional B cells. Germinal centers and cortical areas of lymph nodes are involved in B cell activation and differentiation, not gene rearrangement. While secondary lymphoid organs support B cell activation and immune responses, they are not specifically where the gene rearrangement occurs.

Correct Answer: A

Rationale: Peyer's patches are lymphoid nodules located in the small intestine, specifically in the ileum. They play a crucial role in the immune response of the gut. The correct answer is A: Intestine because Peyer's patches are specifically found in the intestine. The other choices are incorrect because the spleen is a different organ involved in filtering blood, the lungs are primarily involved in respiration, and the skin is the body's largest organ responsible for protection and thermoregulation.