Immune privilege

The immune system patrols nearly the whole body, but there are territories it enters with the brakes on. The eye is the classic example: it is called a site of immune privilege because the systemic immune response against antigens, the substances the body recognizes as foreign, is greatly reduced there.

This is neither accident nor oversight, it is architecture. There are no lymphatic vessels inside the eye, and those are the highways that immune signals and defense cells normally travel along. And the cells of the retinal pigment epithelium are joined to one another by tight junctions, a seal that forms the blood-retinal barrier. As a result, much of what happens inside the eye is never reported to the rest of the body. The hair follicle, the little pouch each hair grows out of, has the same arrangement, and it shields its stem cells from autoimmunity, the situation in which the defenses attack the body's own tissue.

The flip side shows up with an injury that penetrates the eye: uveal and retinal antigens that had been sequestered for a lifetime are exposed to the immune system, and the inflammation that follows is devastating.