Certain skin infections provoke an immunologic response in which eosinophils, mast cells, basophils and IgE antibody-mediated reactions play prominent roles. Such responses are often orchestrated by activated T cells and the production of interleukins such as IL-4 and IL-5. Involvement of the skin by the microfilarial stage of Onchocerca volvulus results in such a response. Onchocerciasis is caused by the filarial nematode O. volvulus. Infective larval stages of the worm are inoculated into human skin during a blood meal of an infected Simuhum blackfly. After migration and maturation of larvae, adult worms become localized in palpable subcutaneous nodules, onchocercomata. Female worms release millions of microfilariae that freely infiltrate the skin and subcutaneous tissues. Involvement of ocular structures can cause keratitis and blindness. Microfilarial involvement of the skin results in significant dermatitis and pruritis. Histopathologic examination of involved skin discloses an eosinophilic infiltrate with extensive extracellular deposition of eosinophil granule proteins. An eosinophilic response in the blood is common. Longstanding infection of skin results in hyper-or hypopigmentary changes, lichenification, skin atrophy and loss of skin elasticity.
An eosinophilic immunologic response is also seen during infection of human skin by the scabies mite, Sarcoptes scabiei. During infection, the female mite burrows through the epidermal cornified layer and deposits eggs and feces. These antigens provoke an immunologic response that results in skin lesions that may be papular, vesicopapular, nodular or diffuse (crusted Norwegian scabies). Although the mites and their products do not penetrate below the epidermis, they provoke a prominent dermal perivascular inflammatory response containing lymphocytes, histiocytes and numerous eosinophils. Diffuse dermal infiltration by eosinophils is not uncommon. The increased propensity of patients coinfected with S. scabiei and human immunodeficiency virus 1 (HIV-1) to have diffuse scabetic involvement of the skin, highlights the importance of cellular immunity in orchestrating an appropriate eosinophilic and IgE-mediated antibody response.
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