Blepharitis: This is an inflammation of the eyelids that can cause redness, itching, and swelling. It's a common condition that can be unilateral and is often associated with other symptoms like crusting or burning sensations.

Allergic Conjunctivitis: An allergic reaction to substances like pollen, dust mites, or pet dander can cause itchy, red eyes and eyelids. This condition is often bilateral but can start unilaterally.
Eczema (Atopic Dermatitis): Eczema can affect any part of the body, including the eyelids, causing redness, itching, and swelling. It's a common condition in individuals with a history of atopic diseases.
Contact Dermatitis: This is a skin reaction that occurs after contact with an irritant or allergen, leading to redness, itching, and sometimes blistering of the eyelid.

Herpes Zoster (Shingles): Although less common, herpes zoster can cause a unilateral, painful rash that may involve the eyelid. It's crucial to diagnose and treat promptly to prevent complications like vision loss.
Basal Cell Carcinoma or Other Eyelid Tumors: While rare, tumors of the eyelid can present with redness, swelling, and sometimes itching. Early detection is vital for effective treatment and to prevent serious complications.
Angioedema: A severe, potentially life-threatening condition that can cause rapid swelling of the eyelids, among other areas. It often requires immediate medical attention.

Sarcoidosis: A systemic disease that can affect various parts of the body, including the eyes. It may cause uveitis, conjunctivitis, or eyelid swelling, among other ocular symptoms.
Lupus: Systemic lupus erythematosus (SLE) is an autoimmune disease that can affect many parts of the body, including the eyes, causing symptoms like dry eyes, sensitivity to light, or eyelid rashes.
Leukemic Infiltration: In rare cases, leukemia can infiltrate the eyelids, causing swelling, redness, and sometimes itching. This would be part of a broader systemic disease presentation.