What underpins most functions of the immune system?

Correct Answer: B

Rationale: The correct answer is B: Haematopoietic stem cells in the bone marrow. These cells are responsible for generating all immune cells, including lymphocytes, macrophages, and dendritic cells. They continuously replenish the immune system, allowing for proper immune function. Secondary lymphoid tissues (choice A) play a role in immune responses but are not the primary underpinning. Mucosal immune system activity (choice C) is important for defense at mucosal surfaces but is not the foundation of immune function. Rapid cytokine suppression (choice D) is a regulatory mechanism, not the fundamental basis of immune system functions.

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.

Correct Answer: C

Rationale: The correct answer is C: Inducing inflammation and recruiting immune cells. Anaphylotoxins, such as C3a, C4a, and C5a, are small molecules that are released during complement activation. They play a crucial role in enhancing inflammation by attracting immune cells to the site of infection or injury. 1. Anaphylotoxins induce inflammation by stimulating the release of histamine and other inflammatory mediators. 2. They promote vasodilation and increase vascular permeability, allowing immune cells to easily reach the affected area. 3. Anaphylotoxins also act as chemoattractants, recruiting neutrophils, macrophages, and other immune cells to the site of inflammation to help fight against pathogens. 4. These molecules do not suppress inflammation (choice A), facilitate antigen presentation (choice B), or neutralize pathogens (choice D). Their primary function is to amplify the immune response by inducing inflammation and recruiting immune cells to combat infections