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The numerous factors that lead to skin allergies on the backside and legs include irritants, allergenic agents, infection or autoimmune disorders. It is necessary to consult a dermatologist that will conduct sufficiently comprehensive evaluation and suggest adequate options for treatment. Self-diagnosis and self treatment can make the situation only worse complicating it.

Such cysts can grow on your neck due to the blockage in the glands. It has been there for an elapsed time and no significant pain has occurred. That is probably nothing to worry about given the duration of its time there and the fact it is asymptomatic. Still, I strongly believe that expert attention from a doctor is needed.

You might have a condition called ichthyosis vulgaris, where the skin becomes dry and itchy because it doesn't shed properly. To manage this, it's important to keep your skin moisturized with non-irritating, fragrance-free lotions. Taking warm, not hot, showers with mild soap can also help. Don’t forget to drink plenty of water, as it helps nourish your skin from the inside.