The different substances that elicit itch upon intracutaneous injection (injection within the skin) elicit only pain when injected subcutaneously (beneath the skin).Itch is readily abolished in skin areas treated with nociceptor excitotoxin capsaicin but remains unchanged in skin areas rendered touch insensitive by pretreatment with anti-inflammatory saponins. Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin. Molecular diversity of itch transmitting primary afferents
Using single-cell mRNA sequencing, sensory-modality specific primary afferent have been molecularly defined into clusters based on gene expression patterns. Here, 11 sub clusters were detected; NF1-3, transmitting innocuous nociceptive information; NF4-5, which transmit proprioceptive information; NP1-3, transmitting itch information; PEP1-2, nociceptive information and TH, which is involved in pleasant touch, The pruriceptive NP1-3 were shown to express genes related to histaminergic and non-histaminergic signaling, where NF1 expresses genes responding to lysophosphatidic acid (Lpar3 and Lpar5), NP2 chloroquine-responsive genes (Mrgpra3 and Mrgprx1), whereas NP3 expresses neuropeptides Nppb and Sst as well as genes involved in inflammatory itch (Il31ra, Osmr and Crystrl2). The histamine receptor gene Hrh1 was found in NP2 and NP3, suggesting that histaminergic itch is transmitted by both these pruriceptive sub clusters. The spinal itch pathway
After the pruriceptive primary afferent has been activated, the signal is transmitted from the skin into the spinal dorsal horn.
Itch inhibition due to pain
Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat, physical rubbing/scratching, noxious chemicals, and electric shock. Causes
Infections
Body louse, found in substandard living conditions
Cutaneous larva migrans, a skin disease
Head lice, if limited to the neck and scalp
Herpes, a viral disease
Insect bites, such as those from mosquitos or chiggers
Pubic lice, if limited to the genital area
Scabies, especially when several other persons in close contact also itch
Shaving, which may irritate the skin
Swimmers itch, a short-term immune reaction
Varicella – i.e. chickenpox, prevalent among young children and highly contagious
Tungiasis, ectoparasite of skin

Environmental and allergic
Allergic reaction to contact with specific chemicals, such as urushiol, derived from poison ivy or poison oak, or Balsam of Peru, found in many foods and fragrances. Certain allergens may be diagnosed in a patch test. Foreign objects on the skin are the most common cause of non-pathological itching. Photodermatitis – sunlight reacts with chemicals in the skin, leading to the formation of irritant metabolites. Urticaria (also called hives) usually causes itching. Skin disorders
Dandruff; an unusually large amount of flaking is associated with this sensation. Punctate palmoplantar keratoderma, a group of disorders characterized by abnormal thickening of the palms and soles. Skin conditions (such as psoriasis, eczema, seborrhoeic dermatitis, sunburn, athletes foot, and hidradenitis suppurativa). Most are of an inflammatory nature. Scab healing, scar growth, and the development or emergence of moles, pimples, and ingrown hairs from below the epidermis.