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Why Mosquito Bites Itch: Saliva Components, Immune Response & Relief

Mosquito bites itch because the female mosquito injects saliva containing >100 proteins during blood-feeding. These proteins prevent clotting, vasodilate, and modulate host immunity—but trigger a robust Type I hypersensitivity reaction in sensitized individuals.

Key Salivary Components Driving Itch

Component	Function for Mosquito	Host Reaction
Anticoagulants (e.g., anophelin, aegyptin)	Inhibit thrombin, Factor Xa, platelet aggregation	Recognized as foreign → IgE binding → mast cell degranulation
Vasodilators (e.g., sialokinin)	Increase blood flow to bite site	Enhances immune cell recruitment; warmth/redness
Immunomodulators (e.g., D7 proteins)	Bind biogenic amines (histamine, serotonin) to suppress host defense	Paradoxically, D7 proteins themselves are major allergens
Enzymes (apyrase, hyaluronidase)	Degrade ADP/ATP (platelet activation); spread tissue	Pro-inflammatory; tissue damage signals

The Itch Pathway

First bite: No reaction (primary sensitization).
Subsequent bites: Salivary proteins cross-link IgE on mast cells/basophils → degranulation → histamine, tryptase, leukotrienes, prostaglandins released.
Histamine binds H1/H4 receptors on sensory neurons (C-fibers) → itch signal via spinal cord → brain.
Late phase (6–24h): Cytokines (IL-4, IL-13) recruit eosinophils, basophils → sustained inflammation, wheal-and-flare.

Why Some People React More

Genetics: HLA, FcεRI variants, filaggrin mutations (barrier defect).
Exposure history: Sensitization requires repeated bites; residents of endemic areas may develop tolerance (IgG4 blocking antibodies).
Skin microbiome: Staphylococcus aureus exacerbates inflammation.
Atopy: Eczema, allergic rhinitis, asthma correlate with larger reactions.

Evidence-Based Relief

Category	Options	Mechanism / Note
Topical (First-line)	Corticosteroids (1% hydrocortisone OTC; prescription mid-potency), Calcineurin inhibitors (tacrolimus), Pramoxine (anesthetic), Menthol/camphor (counterirritant)	Anti-inflammatory; apply BID x 3–5d. Avoid face/groin with potent steroids.
Oral Antihistamines	2nd-gen (cetirizine 10mg, loratadine 10mg, fexofenadine 180mg) preferred—less sedation. 1st-gen (diphenhydramine) for night.	H1 blockade; best if taken early/regularly during season.
Physical	Cold compress (constricts vessels, numbs nerves), Pressure (firm hold 10s), Baking soda paste, Oatmeal bath	Immediate symptomatic relief; safe for kids.
Prevention	EPA-registered repellents (DEET 20–30%, Picaridin 20%, IR3535, Oil of Lemon Eucalyptus), Permethrin-treated clothing, Screens, Eliminate standing water	Most effective strategy. DEET safe >2mo; Picaridin less greasy.

When to Seek Care

Signs of infection: increasing redness, warmth, pus, red streaks, fever.
Systemic reaction: widespread hives, lip/tongue swelling, difficulty breathing, dizziness → anaphylaxis, call 911.
Skeeter Syndrome: large local reaction (>10cm, severe swelling, fever) – may need oral steroids.
Bite in malaria/dengue/Zika/chikungunya endemic area + fever → urgent eval.

Don't scratch. It increases inflammation, infection risk, and scarring. Treat early, prevent bites.