Where in the body are self antigens important? In terms of central tolerance and autoimmunity, but also in terms of T cell activation?
Self-antigens are not recognized by the body. This is due to self-antigens being presented to the immune system of the body during the embryological development. Any immune cell which recognize the pattern are destroyed during the intrauterine period, preventing the emergence of Auto-immunity in the Life-time.
Self-antigens become important in the following situations:
There are self-antigens, called cryptic antigen, which are not presented to the immune system during embryologic development. These antigens generally are not present on the cell surface or they are present behind a barrier which is usually not penetrated by the immune cells. Eg.: The sperm antigens are not usually exposed to immune system as the sperms are behind the blood-testis barrier. Other such barriers are blood brain barrier, blood retina barrier, etc...
- This is important because if for some reason (infection, trauma, etc...) such antigens get into the general circulation they will cause an immune reaction leading to autoimmunity
Self antigens play a role in graft rejection and transfusion reactions. This is mediated specially by the T-cells. This is the reason why transplant patients receive life-long immune supressants and cross-match is done before transfusion
Self-antigen non-recognition also means non-self antigen recognition. This is the basis of cells infected with viruses or tumor cells getting destroyed. As the new protein peptides (viral or mutated regular proteins) presented by the MHC 1 are not recognised as self, such cells are destroyed protecting the body
The corollary of 3 is that any pathogen which resembles the self-antigen are tolerated and active immunity does not come into play i.e. the epitopes that are recognized as self do not cause any immune response
Some pathogens have antigens that closely resemble human antigens. When T-cells mount response against such antigens, a specific sub-clone of T-cell or B-cell (which are activated by the T-cell) may gain the ability to act against the body cells (due to mutations in the hotspots of the anti-topes) that have antigEns resembling the pathogens. This is called molecular mimicry. A well established disease that is due to this mechanism is the Rheumatic Heart Disease. Post-streptococcal Glomerulonephritis is also another example.
There can be many responses. They are important for alloimmunity (so by transfusion, transplantation, etc...), for recognizing pathogens, cancer, virus, etc... By T cell activation, the regulator T cells recognize these antigens as self, and prevent the autoimmune reaction.
I guess they were thinking on MHC1 and MHC2. MHC1 shows the inside of the cells, so e.g. by viral infection or cancer non-self oligopeptides (from degraded proteins) bind to MHC1. These non-self antigens are recognized by CD8+ T cells, which destroy the affected cells. MHC2 is used by APCs to present possible pathogens to CD4+ T cells in a similar way.