Swelling under the right eye may be caused by allergies, mosquito bites, blepharitis and other reasons. 1. Allergy: mainly due to the patient’s contact with allergens, common allergens are house dust, pollen, topical drugs, paint and so on. When allergies occur under the right eye, urticaria can appear, red spot, wind ball-like, can appear under the right eye swelling phenomenon. 2. Mosquito bites: mosquito saliva contains salivary allergens, natural antigenic proteins, adenosine deaminase and other allergy-related proteins, which make the human body after being bitten by local or systemic allergic reactions. Thus, patients may have symptoms such as pimples, wheals, and edematous erythema in the local area of the bite, i.e., swelling under the right eye. 3. Blepharitis: acute suppurative inflammation of the eyelid glands under the right eye may be caused by rubbing the eyes with unhygienic hands or not removing makeup for a long time. Due to the stimulation of inflammation, redness, swelling, heat and pain can occur. It is recommended that patients with swelling under the right eye go to the hospital in a timely manner and follow the doctor’s instructions for treatment to avoid delaying the condition.