We always define antigens are the 'foreign substances' that elicit immune response. Yes, it is true. But, we also have self-antigens that are present on some cells of our body. Surprised? Our body recognizes them as self-antigens and does not attack these.
Assertion: Our blood cells have antigens on them, but our immune system does not attack our blood cells
Reason: Our immune system recognizes the self-antigens of our body and does not attack them.