The potential enemy of the eyeball is actually our own immunity?

There is a saying that our immune system does not know the existence of the eyeball. Once one eyeball is injured and the immune system is activated, the other eyeball may also be "hunted down" by the immune system. This statement sounds a bit incredible, but it is true. Today I will tell you this story about our immune system and eyeballs.

This special situation has a lot to do with "immune immunity", which starts with the working process of our immune system.

Immune response principles and immune immunity

Our immune system is divided into innate immunity and adaptive immunity. Innate immunity is mainly composed of macrophages, neutrophils and natural killer cells. When the human body is invaded, innate immune cells initially resist. At the same time, a type of cell called dendritic cells also rushes to the battlefield, swallows and decomposes pathogens, analyzes the characteristics of the enemy, and then goes to the lymphatic network to find and activate T lymphocytes and B lymphocytes that target this characteristic, thereby mobilizing a large number of targeted and efficient immune cells and antibodies to kill invaders.

In this immune response, the most important part is the immune system's identification of "friend and foe". So how does it do this? From the embryo to the early postnatal period is the maturation period of the human immune system. This "maturation" process is mainly about establishing a "white list" and a "black list". During this period, the normal cells and components of the body that the immune system comes into contact with are written into the white list, while foreign invaders are recorded in the black list, and this list is constantly updated.

Normally, all cells and proteins in our body should be on the "white list", but there are some exceptions. Some parts of our body have natural barriers and special inhibitory mechanisms. Under normal circumstances, the tissue itself or foreign proteins will not come into contact with lymphocytes, nor will they cause immune responses or the immune responses will be very weak, in order to prevent tissue damage and dysfunction caused by immune responses. This phenomenon is called "immune exemption", and these substances that generally do not come into contact with immune cells are called "hidden antigens", and organs with "immune exemption" are called "immune exempt organs".

Dangerous sympathetic ophthalmia

For the eyeball, the most important hidden antigen is the crystallin that makes up the lens. Under normal circumstances, the lens is supplied with nutrients by aqueous humor, has no direct blood supply, and is in a natural barrier.

During the maturation of the immune system, immature lymphocytes cannot come into contact with the lens protein in the eyeball, so the immune system does not recognize the lens protein, and neither includes it in the "white list" nor puts it in the "white list", and of course there are no antibodies against the lens protein.

After the body matures, once the eyeball is severely damaged, the lens protein will enter the blood and meet the lymphocytes. The immune system will identify it as a foreign body and stimulate the body to produce antibodies. These antibodies will circulate throughout the body, including the healthy eye, and eventually produce an inflammatory response, which will damage vision and cause blindness in the other eye in severe cases. This condition in which one eyeball is damaged and causes bilateral granulomatous uveitis is called "sympathetic ophthalmia." In many cases, if the eyeball has been severely injured, the injured eyeball needs to be removed as soon as possible to avoid damage to the other eye and vision.

The picture comes from Tuchong.com

Since this characteristic of the eyeball may cause sympathetic ophthalmia, why does the eyeball still need immune immunity? That is because inflammation and immune response cause tissue edema, cell aggregation and later tissue reconstruction, which will cause the transparent structure in front of the retina to become turbid or even unable to transmit light. The retina and optic nerve are extensions of the central nervous system. Immune and inflammatory responses will seriously affect the formation and transmission of nerve impulses. As we all know, the human immune response is actually happening all the time and maintains a dynamic balance. If immune responses often occur in the eyeball, our vision will often be offline.

The visual information received by a normal person accounts for more than 70% of all sensory information. For us, once the visual function is offline, we will undoubtedly not be able to survive.

Eyes need special protection

The immune system is constantly patrolling the body. It has its own "white list" and uses this "white list" to determine whether the other party is a normal cell. Once it finds that the other party is not on the "white list", it will initiate an immune response to attack it, and at the same time record the characteristics of this "bad guy" in the "black list". Other immune cells will focus on fighting the enemies on the "black list".

Since the immune immunity mechanism of the eyeball has been established during embryonic development, our immune system has never seen this "own person", and this "own person" is not on the "white list" of professional patrol cells. In other words, our immune system does not recognize our eyeballs.

Once a serious injury occurs, the proteins in the eyeball "meet" the immune system, and the immune cells naturally regard this "stranger" as an enemy, and quickly put him on the "blacklist" for special care. As a result, the other normal eyeball is also regarded as a bad guy and "suffers misfortune".

In addition to the eyeballs, the human body's immune-immune organs include the brain, testicles, and uterus during pregnancy. The functions of these parts are very important and fragile. Similar to the eyeballs, once they become battlefields for immune responses, the consequences can be very serious.

Well, that's the end of the story of the eyeball and the immune system. I hope today's content is helpful to you, and I also hope that everyone will protect their eyes in life and work. Especially in the season of fireworks, you must not give the ophthalmologist a chance to make an emergency.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Wang Haoran

Reviewer: Tao Ning (Associate Researcher, Institute of Biophysics, Chinese Academy of Sciences)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.