Introduction
The avian eye’s greater capabilities than the human eyes are attributed to the birds’ unique lifestyle and environments, as well as to physical activities like flight that require excellent vision. The ecomorphological design of the vertebrate eye reflects the characteristics of the species’ environment and lifestyle. Although the morphological aspects of the eye are similar for all vertebrates, its microstructural and molecular features vary among species from different ecological niches. The accessory organs of the eye (eye adnexa) are important for proper functioning of the eye (Bayon et al. 2007). They include the eyelids (upper, lower, and third eyelids), conjunctiva-associated lymphoid tissue (CALT), the Harderian gland (HG), and the lacrimal gland (LG) (Klec´kowska-Nawrot et al. 2015). These structures mainly moisturize the eye and supply the superior and inferior conjunctival sacs with nutrients and antibodies to maintain their health (Dartt, 2009; Maggs, 2013; Jochems & Phillips, 2015; Klećkowska-Nawrot et al. 2016).
Recently, it was recorded that many viruses replicate in the feather pulp and/or the feather follicle epithelium, such as those found on the eyelids (Takashi et al. 2021), for example, Marek’s disease virus (MDV), avian leukosis virus, and chicken infectious anemia virus. Each one of these diseases may cause a partial loss of vision and have far-reaching effects, and the compensatory use of other senses (such as auditory and olfactory sensory perceptions) is typically insufficient (Ahmed et al. 2016)
Ornitho-ophthalmology has played a significant role in this field with very few exceptions e.g., pathogens transmitted via the germline, the animal pathogens enter the host by breaching the barrier between the external and internal milieus. This barrier consists of specialized tissues characterized by two main components: an externally located epithelium and the underlying connective tissue (Korbel, 2000; Korbel & Bohnet, 2007). So, to prevent the entry of pathogens through the mucosal tissues, a wide variety of protective mechanisms have evolved. These range from barrier functions (e.g., keratinized skin) to highly specialized immune cells (e.g., Langerhans cells in the skin) and the organized lymphoid structures in the mucous membrane. Obviously, the epithelial cells of the mucosal surfaces are able to sense pathogens and actively shape the response of the immune cells underneath (Alan & Athena, 2019).
For many years, research in mucosal immunology has focused on the question of how these cells become activated and interact to protect the host from invasion and dissemination of pathogenic microorganisms (Yasmine & Timothy, 2014). Under physiological conditions, the mucosal immune system has to remain tolerant of the microbiome to avoid chronic mucosal inflammation, while at the same time being able to respond quickly and appropriately to pathogenic challenges. This explains the great importance of the eyelids in protecting the eye from diseases and that they are not only responsible for controlling the amount of light and moisturizing the eye but also play an important role in protecting the eye from diseases (Knop N & Knop E 2005 a &b).
The present authors selected a common wild bird species in my country, the black winged kite to investigate the morphological features and microstructure as well as demonstrate the lymphoid follicles and lymphocytes to illustrate the importance of the characterization of the eye adnexa in the protecting of the eye from any diseases.