Why does lice cause itching? - briefly
Lice bite the scalp and inject saliva that contains proteins which trigger an allergic response and histamine release, producing the characteristic itching.
Why does lice cause itching? - in detail
Lice provoke itching primarily through the injection of saliva while feeding on blood. The saliva contains anticoagulants and enzymes that prevent clotting and facilitate nutrient extraction. These foreign proteins are recognized by the host’s immune system as antigens, triggering a localized hypersensitivity reaction. Mast cells in the skin release histamine and other inflammatory mediators in response, producing the characteristic pruritic sensation.
The inflammatory cascade also involves cytokines such as interleukin‑1, interleukin‑6, and tumor necrosis factor‑α. These substances increase vascular permeability, leading to swelling and redness around the bite site. The combination of histamine‑induced nerve irritation and cytokine‑mediated inflammation amplifies the itch signal transmitted to the central nervous system.
Repeated bites exacerbate the response. Continuous exposure to lice saliva sensitizes the immune system, resulting in a heightened reaction known as a delayed‑type hypersensitivity. Consequently, even minor feeding events generate pronounced itching and may develop into papular or pustular lesions.
Secondary factors contribute to the discomfort. Scratching damages the epidermal barrier, allowing bacterial colonization and potential infection. Bacterial products further stimulate inflammatory pathways, prolonging the itch cycle. Additionally, the mechanical movement of lice across the scalp can irritate hair follicles, adding a tactile component to the sensation.
Key mechanisms can be summarized:
- Saliva‑borne antigens → mast cell activation → histamine release.
- Cytokine production → vasodilation and edema.
- Sensitization from repeated exposure → intensified immune response.
- Physical trauma from scratching → barrier disruption → bacterial invasion.
- Direct mechanical irritation of follicles → additional sensory input.
Understanding these processes clarifies why infestations are associated with persistent pruritus and underscores the importance of prompt removal and treatment to interrupt the cycle of irritation and inflammation.