Abstract: Novel antibodies and antigen binding fragments that specifically binds to Siglec-15 are described herein In some embodiments, the antibodies or antigen binding fragments may block the biological activity of Siglec-15 and are useful in composition for the treatment of bone loss, more particularly in bone diseases that have increased cell surface expression of Siglec-15, such as conditions where there is an increase in the bone degradative activity of osteoclasts The invention also relates to cells expressing the antibodies or antigen binding fragments such as monoclonal, humanized or chimeric antibodies Additionally, methods of detecting and treating bone loss, bone-related diseases or cancer using the antibodies and fragments are also disclosed.