How similar do proteins have to be to trigger the same immune response?
Immunoglobulins don't recognize entire proteins, but rather,specific shapes (called epitopes). These epitopes must also be present on the surface of the invading bacteria or virus for the immunoglobulin to have access. The epitope must be exactly the same. Any change in the amino acid sequence would change the shape. But an epitope is a relatively small part of the larger protein. If the shape is the same between different proteins, it would trigger an immune response.