Why the Immune System Doesn't Protect Against Head Lice

Head lice (Pediculus humanus capitis) are ectoparasites (insects), not microorganisms. They live on the scalp, feed on blood, and attach eggs (nits) to hair shafts.

The immune system recognizes antigens (proteins) from pathogens (bacteria, viruses, fungi, protozoa). Lice are multicellular eukaryotes; their presence does not elicit a protective adaptive immune response that prevents infestation. Local reactions (itching) are due to sensitization to louse saliva anticoagulants, not immunity that clears the parasite.

Treatment requires physical removal (wet combing) and/or pediculicides (permethrin, malathion, ivermectin, spinosad). Environmental decontamination of fomites (bedding, brushes) prevents reinfestation.