The causes of red rash on the neck are: hives, eczema, prickly heat and so on. 1. Urticaria: for people with sensitive body, when the neck is in contact with pollen, willow, dust and other allergens in the environment, it is easy to develop urticaria, which manifests itself as the symptom of a red rash on the surface of the skin and mucous membranes, and it is a common and highly prevalent skin disease. 2. Eczema: It is related to heredity, chronic infection, endocrine diseases and allergen stimulation. When eczema appears on the neck, it will also be manifested as a small red rash, accompanied by itching, molting and other manifestations. 3. prickly heat: often appear in the hot and humid environment in summer, prickly heat on the neck will appear as a dense small red rash, usually accompanied by itching, burning sensation, and aggravated with sweating. As you can see, it is recommended to consult a doctor in time to treat the symptoms of the rash on the neck under the professional guidance of the doctor.