What is the central component of the complement system?

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: 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

Correct Answer: B

Rationale: The correct answer is B: Cytotoxic T cells. Cytotoxic T cells are responsible for directly killing infected or foreign cells by releasing toxic substances that induce apoptosis in the target cells. This process helps eliminate the threats to the body. Helper T cells (Choice A) assist in coordinating immune responses but do not directly kill cells. Regulatory T cells (Choice C) play a role in suppressing immune responses to prevent autoimmunity but do not kill infected cells. Memory T cells (Choice D) are primed for rapid response upon re-exposure to a specific antigen but do not directly kill cells. Therefore, the correct choice is B as cytotoxic T cells are specifically designed for cell killing.