Self vs. non-self immunity

How do our immune cells recognize self and non self?
In the Immune system, when the B cell and T cells mature there is a process known as a Central Tolerance, where auto-reactive cells, which can attack self antigens are selectively eliminated. Similar process also happen in case of peripheral tolerance. So, when the immune cell mature they tend to recognize self antigens as "self" and they do not engage or harm them. Most importantly the T cells are not reactive to self antigens presented by self MHCs. 

Now, when a foreign antigen enters the body, they are recognized by means of various proteins like antibodies,  complement proteins and innate immunity receptors like TLRs (Toll like receptors) and finally, cleared by the phagocytes.

Finally, cells of the immune system recognize antigens to which they are exposed. So, conversely, if a self antigen to which these cells are not exposed because the antigen was hidden/sequestered, then whenever that antigen is presented, it may be recognized as non self. Why, because immune cells never got acquainted to that antigen. example of such antigens, which never get exposed to the immune cells and are always hidden are MBP, sperms and eye lens 

https://www.quora.com/How-do-our-immune-cells-recognize-self-and-nonself

antigens.https://www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology/introduction-to-immunology/v/self-versus-non-self