Which of the following is a safety mechanism of the complement system?

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: 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.

Correct Answer: B

Rationale: The correct answer is B: Affinity maturation. During a secondary immune response, B cells undergo somatic hypermutation, leading to the production of antibodies with higher affinity for the antigen. This process occurs in germinal centers within secondary lymphoid organs. Isotype switching (A) refers to the change in antibody class but not affinity. Somatic recombination (C) is the process of creating diverse antibody repertoires. Clonal expansion (D) involves the proliferation of antigen-specific B cells but does not directly address affinity enhancement.

Correct Answer: B

Rationale: The correct answer is B: Helper T cells. Helper T cells play a crucial role in activating B cells by releasing cytokines that stimulate B cell proliferation and differentiation into plasma cells, which produce antibodies. Cytotoxic T cells (A) are involved in directly killing infected cells. Regulatory T cells (C) suppress immune responses. Memory T cells (D) are responsible for maintaining immunological memory for future responses. Thus, Helper T cells are the key cell type that helps activate B cells.

Correct Answer: C

Rationale: Regulatory T cells (Tregs) maintain immune homeostasis by suppressing immune responses using IL-10. Tregs release IL-10, which inhibits the activity of effector T cells and other immune cells, preventing excessive immune responses and maintaining balance. This mechanism helps prevent autoimmune diseases and chronic inflammation. Choices A, B, and D are incorrect because Tregs do not kill infected cells, produce pro-inflammatory cytokines, or activate macrophages. Instead, Tregs focus on regulating and dampening immune responses to maintain overall immune balance.