Most of the redness and swelling of the arm after DPT vaccination is the adverse reaction of the vaccine, which is normal, and some of them may be caused by allergy, infection and other abnormalities. 1. Adverse reaction: DPT vaccine belongs to a kind of biological agent, which is a foreign substance to the human body, and it will produce immune reaction when injected into the body, which can cause local inflammatory reactions such as redness, swelling and pain. Therefore, most of the redness and swelling of the arm after DPT vaccination is normal. 2. Infection: After taking DPT vaccine, the local wound care is not done well, which makes the pathogenic bacteria invade into the body and cause infection, which can also lead to redness and swelling of the arm. This is not normal, and the wound needs to be treated. 3. Allergy: If there is redness and swelling of the arm after vaccination, and there is also rash, itching, even swelling of the lips and difficulty in breathing, it is abnormal and needs to be dealt with in time, and serious allergic reaction is forbidden to be vaccinated again. If your arm is red and swollen after vaccination, you need to actively consult a professional doctor and handle the case under the guidance of a professional doctor.