Correct Answer: C. IL-1 is an endogenous pyrogen
Interleukin-1 (IL-1) is a canonical endogenous pyrogen — a cytokine produced by activated macrophages, monocytes, and dendritic cells during infection or inflammation that acts on the hypothalamic thermoregulatory centre to raise the set-point temperature. IL-1 does not directly raise body temperature; instead, it stimulates hypothalamic neurons to synthesize and release PGE2 (not PGD2), which resets the anterior hypothalamic temperature set-point upward. This is why fever is a regulated response, not uncontrolled heat generation. Other endogenous pyrogens include TNF-α, IL-6, and IFN-γ. In Indian clinical practice, understanding endogenous pyrogens is critical for interpreting fever in sepsis, tuberculosis, and other chronic infections where IL-1 drives the systemic inflammatory response. The distinction between exogenous pyrogens (bacterial lipopolysaccharides, toxins) and endogenous pyrogens (host cytokines) is fundamental to pathophysiology and explains why antipyretics work downstream of IL-1 action.
Why the other options are wrong
A. Aspirin increases fever by inhibiting prostaglandin synthesis — This is backwards. Aspirin and NSAIDs reduce fever by inhibiting cyclooxygenase (COX), thereby blocking PGE2 synthesis in the hypothalamus. PGE2 is the final mediator that raises the temperature set-point; blocking its production lowers fever. This is a classic NBE trap that reverses the mechanism to test mechanistic understanding. B. PGD2 is responsible for resetting the temperature to a higher level — PGD2 is not the thermoregulatory prostaglandin — PGE2 is. PGE2, not PGD2, acts on EP3 receptors in the anterior hypothalamus to raise the temperature set-point. PGD2 is involved in sleep-wake regulation and immune suppression. This option confuses prostaglandin subtypes to trap students who know prostaglandins are involved but lack specificity. D. Fever occurs during inflammation due to release of histamine — Histamine is a mediator of acute inflammation (vasodilation, increased vascular permeability) but is not a pyrogenic mediator. Fever in inflammation is driven by IL-1, TNF-α, and IL-6, not histamine. This option conflates acute inflammatory mediators with pyrogenic cytokines — a common misconception in Indian medical education where histamine is heavily emphasized in allergy chapters.
High-Yield Facts
- IL-1, TNF-α, IL-6, and IFN-γ are the four main endogenous pyrogens; IL-1 is the prototype.
- PGE2 (not PGD2) is the final hypothalamic mediator that resets the temperature set-point upward via EP3 receptors.
- Aspirin and NSAIDs reduce fever by blocking COX → ↓ PGE2 synthesis in the hypothalamus.
- Endogenous pyrogens are host-derived cytokines; exogenous pyrogens are bacterial/microbial products (LPS, peptidoglycans).
- Fever is a regulated hyperthermia (set-point raised), not uncontrolled heat production — this explains why patients with fever still shiver when cold.
Mnemonics
Endogenous Pyrogens: TILI TNF-α, IL-1, Lymphokines (IL-6), IFN-γ — the four main host cytokines that trigger fever. IL-1 is the most studied and the prototype endogenous pyrogen. Fever Pathway: IL → PGE → ↑ Set-point IL-1 (endogenous pyrogen) → hypothalamic neurons → COX → PGE2 → EP3 receptors → temperature set-point raised. Aspirin blocks COX, so it breaks the chain and reduces fever.
NBE Trap
NBE pairs "prostaglandin" with "PGD2" to trap students who know prostaglandins are involved in fever but confuse the subtype. Similarly, option A reverses the aspirin mechanism to test whether students understand that NSAIDs reduce rather than increase fever.
Clinical Pearl
In Indian TB wards and sepsis units, fever driven by IL-1 is why patients with active tuberculosis or bacterial infection respond to antipyretics (paracetamol, ibuprofen) but the underlying infection persists — treating fever does not treat the cause. This distinction guides clinical decision-making: suppress fever for comfort, but always investigate and treat the source of endogenous pyrogen release.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 2 (Inflammation); Harrison's Principles of Internal Medicine, Ch. 7 (Fever)_