Q.Cornea transplant in humans is almost never rejected. State the reason.

The eye, held in the orbit by six extrinsic muscles (superior, inferior, lateral and medial rectus, plus superior and inferior oblique), is protected by eyelids, eyelashes, ciliary glands and lacrimal glands (which secrete tears containing salts, mucus and lysozyme) and covered externally by the conjunctiva. Its wall has three layers: the outer sclera (with the transparent cornea at the front and the canal of Schlemm draining aqueous humor at the corneoscleral junction), the vascular pigmented choroid (thickening anteriorly into the ciliary body and the iris, whose radial and circular muscles control pupil size), and the inner retina, which contains photoreceptor cells (rods and cones), bipolar cells and ganglion cells, with the macula lutea/fovea centralis (cone-only, sharpest vision) and the photoreceptor-free blind spot where the optic nerve exits. Vision works by light refracting through the cornea, aqueous humor and lens onto the retina, dissociating retinal from opsin in the rod/cone photopigment and generating an action potential rel …