T lymphocytes are an integral part of the immune system that help destroy invading pathogens. Understanding how they discriminate between foreign- and self-antigens could prove to be the key to treating autoimmunity.
T lymphocytes that recognise self-antigens get eliminated in the thymus through a process known as negative selection. When this process goes wrong, self-reactive T cells can reach and potential destroy various peripheral organs in the body (e.g. pancreas) and consequently cause autoimmune conditions such as diabetes.
Further details: The mis-education of T cells