Why does the head itch with lice? - briefly
Lice feed on scalp blood, injecting saliva that provokes an allergic response and irritation, which produces itching. Their presence and movement also mechanically stimulate nerve endings, adding to the discomfort.
Why does the head itch with lice? - in detail
Lice attach to the scalp and feed on blood. During feeding they inject saliva that contains anticoagulants and proteins. The host’s immune system recognizes these substances as foreign, triggering a localized inflammatory response. Histamine release causes the characteristic pruritus.
Mechanical irritation also contributes. The tiny claws of adult lice and the friction of moving insects stimulate cutaneous nerve endings. Repeated movement of lice across the skin amplifies the sensation of itching.
Secondary factors intensify discomfort. Scratching damages the epidermis, creating micro‑abrasions that become entry points for bacteria. Bacterial colonization provokes additional inflammation, further aggravating the itch.
The itch cycle can be summarized as follows:
- Salivary antigens → immune activation → histamine → itch.
- Physical contact → nerve stimulation → itch.
- Scratching → skin breach → bacterial infection → heightened inflammation → stronger itch.
Understanding these mechanisms clarifies why a lice infestation reliably produces scalp itching. Effective treatment must eliminate the insects, reduce inflammatory mediators, and prevent secondary infection to break the cycle.