The classic 'rigor' occurs when the body, usually in response to exogenous pyrogens, inappropriately attempts to 'reset' core temperature to a higher level. This latter varies; hence the cyclical shivering followed by sweating, tachycardia and flushing as the body attempts to regain normal temperature control. Exogenous pyrogens are molecules which interact with host cells to induce the secretion of pyrogenic cytokines. Most act directly on monocytes or macrophages.