What is self-immunoreactivity?
The basic concept of the immune system is that it has to discriminate between self and foreign and that is only allowed to launch an immune reaction against something foreign. This is called immune tolerance which basically makes sure that all immune cells which are directed against the own organism are destroyed.

If these antibodies survive, they can attack the own body (and are hence called self immunoreactive) and cause damage. Self-immunoreativity is a reaction of the body against itself. Problems with the immune tolerance are called autoimmune diseases, examples would be  systemic lupus erythematosus, rheumatoid arthritis or type 1 diabetes.