Blood allergy essentially means an allergic body that is prone to an excessive immune response to external and self-substances, leading to a range of symptoms. Blood allergies have a variety of causes. 1. Autoantibody formation is one of the more common causes. In addition, external infections are also one of the common causes of blood allergies. 2. External infections including bacteria, viruses, parasites, etc. can cause blood allergies. Allergic people can also be allergic to certain foods and drugs. For example, some people consume seafood, such as fish, shrimp and crab, and some people consume milk, eggs and mutton, which can cause blood allergy symptoms. Some people who inhale outside substances, such as pollen and paint, can also develop allergic symptoms. Therefore, there are various causes of blood allergy, mainly related to one’s own constitution, infection, external substances, food, etc.