Introduction
The ocular immune system guards against infection and controls the healing process after damage. The uvea, which is the area of the eye that houses numerous immune cells, primarily macrophages, dendritic cells, and mast cells, is highly vascularized but devoid of lymphatic arteries. These cells protect the eyes from infections, and inflammation within the eye might appear as uveitis (including iritis) or retinitis.
The cornea of the eye is a particularly unique tissue. Due to its continual contact with the outside environment, it is open to various bacteria, with the cornea being particularly sensitive due to its wet mucosal surface. At the same time, immunological defense is challenging because it lacks vasculature and relative immune isolation from the rest of the body. The cornea is also a tissue with many different functions. It is a barrier to prevent viruses from entering the rest of the eye, similar to how the dermis and epidermis protect underlying tissues. It contributes a significant portion of the eye's refractive power, requiring it to retain exceptional transparency.
What Are the Immune Difficulties for the Cornea?
The cornea's primary role is to transmit and refract light, enabling the production of clear, high-resolution pictures on the retina's back. To do this and lessen light dispersion, the collagen in the cornea is highly organized, having a diameter of 30 nanometers and being spaced 60 nanometers apart.
In addition, the tissue lacks vascularization and, except for a few dendritic cells (DC), lacks lymphoid cells and other defense systems. The cornea's sparse cell population is necessary for both causes. As a result, there is a delay between exposure to a pathogen and building an immune response. However, this requires maintaining immune cells at a relative distance. As a result, non-local sources like the conjunctiva are responsible for several immunological and defensive responses within the cornea, such as feeding and moisturization.
What Are the Immune Responses of the Cornea?
Unprejudiced defense is provided by innate immune responses against infections and toxins. In addition to acting as a main line of defense from birth, they offer an innate barrier against corneal infection. The orbit and the eyelid can protect the body from stressful occurrences and outside debris that could harbor bacteria. Tears, epithelial cells, keratocytes, corneal nerves, the complement system, and interferons are included in the list of elements that comprise the ocular innate immune system.
Acquired immune responses are far more pathogen-specific than their innate immunological counterparts. It is known that the cornea's Langerhans cells have a role in some of these cell-mediated pathways' regulation. These Langerhans cells serve as antigen-presenting cells, which collect fragments of encroaching pathogens and use them to trigger an immune response. Cell-mediated immune responses have a considerably delayed onset time but are more effective. However, they can harm nearby tissues, harming the eye's ability to see.
What Are Lacrimal Immune Responses?
Three layers make up the tear film: lipid, aqueous, and mucin. These aid in refraction by providing a smooth surface, lubricating eyelid motion, passively transferring gases like oxygen and carbon dioxide, and shielding the cornea. The tear film's numerous layers all play a part in its final function. The moist environment created by tears shields corneal epithelial cells from drying out and deterioration. But lysozymes, lactoferrins, lipocalin, and beta-lysine, which support pathogen defenses like lysis of bacterial cell walls, prevention of bacterial and viral binding, inflammation, and detoxification, are also present in the liquid layer of the tear film, giving it additional antimicrobial properties.
In addition, the tear film can transfer white blood cells to the ocular surface and dilute and wash away debris and harmful substances. Immunoglobulins, particularly IgA, are also present in the tear film at substantially higher amounts than in serum. Bacterial binding has been demonstrated to be prevented by IgA. IgA may also neutralize viruses and attach to bacteria, assisting their identification via other pathways. It is found in the tear film together with another immunoglobulin called IgG.
Corneal Epithelial Cells
The interior of the eye chamber, which is physically isolated from the rest of the body by tight junctions, is protected from germs by a physical barrier provided by the corneal epithelial cells. To stimulate microbial defense, ocular epithelial cells also release cytokines concurrently. When the cell membrane is torn by infection or trauma, the cytokine interleukin (IL)-1 is immediately released from storage in epithelial cells. However, IL-1's long-term effects can also result in neovascularization (the development of new blood vessels), which can reduce corneal transparency and increase immune infiltration of the cornea. As a result, it has also been discovered that the cornea secretes IL-1RN, an IL-1 antagonist that inhibits neovascularization and reduces leucocyte infiltration.
Corneal Keratocytes
The stroma of the cornea contains flattened cells called keratocytes. It is believed that maintaining the collagen lamellae-covered extracellular matrix is the principal function of this small group of cells. However, during the pathogenic invasion, keratocytes also function defensively. They can generate IL-6 and defensins in response to IL-1 (expressed by corneal epithelial cells) and tumor necrosis factor (TNF). Both the co-stimulation of other immune components and the release of antibodies are observed to be enhanced by the former of these interleukins when it interacts synergistically with other interleukins. The latter, known as defensins, have several antimicrobial activities against bacteria, fungi, and viruses and properties that hasten the repair of injured epithelial cells. Defensins released by corneal keratocytes have also been discovered to be associated with instances of corneal transplant rejection, indicating that these peptides may play a part in tissue rejection.
Corneal Nerves
Corneal nerves provide a line of defense by identifying foreign objects on the ocular surface. This results in reflexive responses such as enhanced lacrimal secretion, blinking, and the production of neuropeptides, which can activate cytokines.
Conclusion
Organs, tissues, and cells that make up the immune system in the body function to defend us against illness and infection. The immune system activates when a virus or other foreign material is found. It produces antibodies to combat these foreign compounds, also known as antigens. A natural defense mechanism, an inflammatory reaction, causes tissue swelling and a higher-than-normal body temperature. Inflammation can aid in the defense against illness or infection but can also lead to issues. Only a few bodily parts have immune privileges, including the eye. The eye controls its inflammatory immune response to prevent vision loss due to swelling and other tissue changes.
