Fever occurs during inflammation due to all except:

Explanation

## Correct Answer: D. Histamine Histamine is a preformed mediator released from mast cells and basophils during inflammation, but it does NOT induce fever. Fever during inflammation is mediated by **pyrogenic cytokines** — primarily IL-1, TNF-α, and IL-6 — which act on the hypothalamic thermoregulatory centre to raise the set-point temperature. These cytokines trigger the production of **prostaglandin E2 (PGE2)** in the hypothalamus, which is the final common pathway for fever induction. Histamine's role in inflammation is limited to vasodilation, increased vascular permeability, and itching — it has no thermogenic effect. In Indian clinical practice, understanding pyrogenic mediators is critical for managing sepsis and post-operative fever, where NSAIDs (which block PGE2 synthesis) are used to reduce fever. Histamine antagonists (H1 and H2 blockers) do not reduce fever, confirming histamine's lack of pyrogenic activity. ## Why the other options are wrong **A. IL-1** — IL-1 (interleukin-1) is a **primary endogenous pyrogen** released by activated macrophages and monocytes. It is one of the most potent fever-inducing cytokines and acts on the hypothalamus to increase PGE2 synthesis, raising the temperature set-point. This is a cardinal cause of fever in infection and inflammation. **B. TNF** — TNF-α (tumour necrosis factor-alpha) is a major **endogenous pyrogen** secreted by macrophages and T cells. It synergizes with IL-1 to induce fever by stimulating hypothalamic PGE2 production. TNF is a key mediator of fever in bacterial infections, sepsis, and inflammatory conditions — a high-yield concept in Indian medical exams. **C. Prostaglandins** — **Prostaglandin E2 (PGE2)** is the final common pathway for fever induction in the hypothalamus. IL-1 and TNF act upstream to trigger PGE2 synthesis; PGE2 then resets the hypothalamic thermostat. NSAIDs reduce fever by blocking PGE2 synthesis — a clinically essential mechanism in Indian practice. ## High-Yield Facts - **Endogenous pyrogens** (IL-1, TNF-α, IL-6) act on the hypothalamus to trigger PGE2 synthesis and raise the temperature set-point. - **Prostaglandin E2 (PGE2)** is the final common mediator of fever in the hypothalamus; NSAIDs block its synthesis. - **Histamine** causes vasodilation and increased vascular permeability but has NO pyrogenic activity — it does not induce fever. - **Macrophages** are the primary source of IL-1 and TNF-α in response to pathogen-associated molecular patterns (PAMPs). - **H1 and H2 receptor antagonists** do not reduce fever because histamine is not a pyrogenic mediator. ## Mnemonics **PYRO-CYTOKINES** **IL-1, TNF, IL-6** → Hypothalamus → PGE2 ↑ → Fever. Remember: **I**nterleukin-**1**, **T**umour **N**ecrosis **F**actor, **I**nterleukin-**6** are the 'big three' pyrogenic cytokines. Histamine is NOT in this list. **NSAID Mechanism in Fever** NSAIDs block **COX → ↓PGE2 → ↓Fever**. If a drug reduces fever, it must work upstream (blocking cytokine release) or downstream (blocking PGE2). Histamine does neither. ## NBE Trap NBE pairs histamine with other inflammatory mediators to test whether students confuse histamine's vasodilatory effects with pyrogenic activity. The trap is assuming "all inflammatory mediators cause fever" — histamine is a classic exception that must be memorized. ## Clinical Pearl In Indian clinical practice, a febrile patient with elevated IL-6 and TNF-α (e.g., in sepsis or post-operative inflammation) will respond to NSAIDs or paracetamol, which block the PGE2 pathway. Antihistamines will NOT reduce fever, a key bedside distinction when managing hospitalized patients. _Reference: Robbins Ch. 2 (Inflammation); Harrison Ch. 7 (Fever)_