The Role of the Eyelids of the Black-Winged Kite, Elanus caeruleus in the Immune Protection of the Eye.

This study aimed to describe the morphological features and microstructure of the upper, lower, and third eyelids of the black-winged kite, Elanus caeruleus, and to characterize the organized lymphoid follicles and lymphocytes in the eyelid mucosa. Additionally, it aimed to illustrate the importance of the eye adnexa in the eye's immune protection. The upper, lower, and third eyelids display varying morphological differences that seem to be closely linked to the birds' way of life, indicating adjustments to their environment and eating behaviors. The black-winged kite has large, forward-facing eyes positioned under a bony shelf (lacrimal process) that shades them. Both eyelids have thick, pigmented edges and bear two rows of long, finely modified filoplume feathers, which increase at the anterior canthus. Melanocytes appear in the stratum basale of upper and lower eyelids, and Langerhans cells were observed. Aggregations of lymphatic cells were present under the conjunctival epithelium within the stroma of the lower eyelid (in the orbital zone near the tarsal plate), while they were absent in the upper eyelid and nictitating membrane. The density of goblet cells on the conjunctival surface of the upper eyelid and the nictitating membrane is higher than that of the lower eyelid. The structure of the skin and conjunctiva in both the upper and lower eyelids contains numerous defensive immune cells that maintain ocular safety and hydration. Additionally, the lower eyelid's CALT constitutes most of the CALT tissue in E. caeruleus and is recognized as part of the mucosal immune system.