The back of the head protrudes and has itching. This condition is most commonly caused by mosquito bites, which can cause localized bumps due to allergies and localized itching. Then there is sebaceous cysts. The so-called sebaceous cyst is the sebaceous glands in the normal secretion of sweat, sweat can be discharged to the body through the small ducts. However, if the ducts are blocked, they continue to secrete sweat, causing a gradual increase in size, resulting in a mass at the back of the head, sometimes accompanied by local itching.