Immune tolerance (self vs non-self)
The mechanisms that teach the immune system to ignore the body's own molecules — and the reason neoantigens are attractive while autoimmunity is the risk.
During development, T cells whose receptors bind self-peptides too strongly are deleted or restrained (central and peripheral tolerance). This is why you don't normally attack your own tissue — and why most self-antigens make poor vaccine targets: the reactive T cells were already removed.
Neoantigens slip past this: arising from somatic mutations, they were never part of the self-image the immune system learned, so a tolerance-shaped T-cell repertoire against them can still exist. The flip side is the central safety constraint — a predicted neoantigen that is actually too close to a self sequence risks triggering autoimmunity, so similarity-to-self is an explicit filter in selection.