How do regulatory T cells (Tregs) maintain immune homeostasis?

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.

Correct Answer: B

Rationale: The correct answer is B: Macrophages. Macrophages are key immune cells involved in granuloma formation. They play a crucial role in engulfing and processing pathogens or foreign particles, leading to the formation of granulomas as part of the body's immune response. Mast cells (A) are primarily involved in allergic reactions. Neutrophils (C) are important in the initial immune response to infections but are not typically associated with granuloma formation. Dendritic cells (D) are important in antigen presentation and activation of T cells, but they are not the primary immune cells involved in granuloma formation.

Correct Answer: B

Rationale: The correct answer is B: C3a and C5a. In Type III hypersensitivity reactions, immune complexes form and deposit in tissues, leading to complement activation. C3a and C5a are anaphylatoxins released during complement activation, causing inflammation and tissue damage in Type III hypersensitivity. Rationale: A: C1 and C4 are primarily involved in Type I hypersensitivity reactions. C: C5b and C6 are part of the membrane attack complex in the classical pathway of complement activation, not specific to Type III hypersensitivity. D: C2 and C7 are involved in the classical pathway of complement activation but not directly implicated in Type III hypersensitivity reactions.

Correct Answer: B

Rationale: The correct answer is B: Among the highest globally. Sarcoidosis prevalence in Ireland is relatively high compared to other regions. This is due to genetic factors, environmental influences, and possibly healthcare practices. Sarcoidosis rates vary worldwide, with Ireland being noted for its higher prevalence. Choices A, C, and D are incorrect because Ireland's sarcoidosis prevalence is not among the lowest globally, similar to the global average, or extremely rare.

Correct Answer: A

Rationale: The correct answer is A: Discoid rash on skin exposed to sunlight. Systemic lupus erythematosus (SLE) commonly presents with a characteristic butterfly-shaped rash on the face, especially over the cheeks and nose, that worsens with sunlight exposure. The rash is not limited to skin exposed to sunlight. Choices B, C, and D are inconsistent with SLE. B: Urinalysis negative for casts and protein is not indicative of SLE, as renal involvement in SLE often presents with the presence of casts and protein in the urine. C: Painful, deformed small joints is more typical of rheumatoid arthritis, not SLE. D: Pain on inspiration is a symptom of pleurisy, which can occur in SLE but is not a defining feature.