What is the primary function of lymph nodes?

Correct Answer: B

Rationale: The primary function of lymph nodes is to filter lymph and facilitate immune responses. Lymph nodes contain immune cells that help recognize and fight off infections. They filter lymph by trapping and removing pathogens and foreign particles. This process helps in activating the immune response to protect the body. Choices A, C, and D are incorrect because lymph nodes do not produce red blood cells, synthesize antibodies, or generate cytokines. These functions are carried out by other organs and cells in the body.

Correct Answer: C

Rationale: The correct answer is C: C3. C3 is the central component of the complement system as it plays a crucial role in both the classical and alternative pathways by forming the C3 convertase enzyme. This enzyme initiates the cascade of reactions leading to opsonization, chemotaxis, and cell lysis. Choices A, B, and D are incorrect as C1 is the initiator of the classical pathway, C2 is involved in the formation of the C3 convertase, and C5 is necessary for the formation of the membrane attack complex.

Correct Answer: B

Rationale: The correct answer is B because pattern recognition receptors (PRRs) are encoded in the germline, meaning they are genetically determined and do not change during an individual's lifetime. This ensures consistency in detecting various pathogens across different individuals. Choices A, C, and D are incorrect because PRR diversity is not influenced by somatic recombination, does not change constantly in response to infections, and is not specific to individual antigens.

Correct Answer: B

Rationale: Individuals with complement deficiencies are more susceptible to encapsulated bacteria. Haemophilus influenzae is an encapsulated bacterium, making it the correct choice. The complement system plays a crucial role in opsonization and killing of encapsulated bacteria. Escherichia coli (choice A) is not an encapsulated bacterium, so complement deficiency doesn't significantly increase susceptibility. Staphylococcus aureus (choice C) and Mycobacterium tuberculosis (choice D) are also not encapsulated bacteria, therefore complement deficiencies do not have a direct impact on susceptibility to these pathogens.

Correct Answer: C

Rationale: Rationale for Choice C: Covalent binding of C3b and C4b to pathogen surfaces is a safety mechanism of the complement system because it helps in targeting pathogens specifically, enhancing opsonization, and preventing damage to host cells by limiting complement activation to the pathogen surface. C3b and C4b form stable covalent bonds with the pathogens, marking them for destruction by phagocytes. This mechanism ensures that complement activation is directed towards the pathogen and not host cells, thus maintaining immune homeostasis. Summary: A: Binding C5a to plasma inhibitors is not a safety mechanism but a regulatory mechanism in the complement system. B: Activation of complement only in the absence of pathogens is incorrect as the complement system can be activated in response to various stimuli, not just pathogens. D: Suppression of C1 activation by antibodies is not a safety mechanism but a regulatory mechanism involving the classical pathway of complement activation.