Certain cells, including types of cancer cells such as lymphomas, are capable of expressing high levels of CD84Hy1. Immunotargeting using CD84Hy1 polypeptides, nucleic acids encoding for CD84Hy1 polypeptides and anti-CD84Hy1 antibodies provides a method of killing or inhibiting that growth of CD84HY1Protein-expressing cancer cells. Methods of immunotherapy and diagnosis of disorders associated with CD84Hy1 protein-expressing cells are described.