How do the white blood cells tell the difference between your own body's cells' and invading pathogens?

Invading pathogens contain specific proteins present on their surface known as antigen. These antigens are recognized by white blood cells in the body. Upon recognition by complimentary binding, the white blood cells are able to kill the invading pathogen. However, normal body cells contain cell surface receptors which are not recognized as foreign or harmful by white blood cells and hence not attacked by them.