The present invention relates to antibodies and related molecules that immunospecifically bind to Tumor Necrosis Factor Delta (TNF-delta; APRIL). The present invention also relates to methods and compositions for detecting, diagnosing, prognosing, treating, preventing, or ameliorating a disease or disorder associated with aberrant APRIL or APRIL receptor expression or aberrant function of APRIL or APRIL receptor, comprising antibodies or fragments or variants thereof, or related molecules, that immunospecifically bind to APRIL. In particular, the present invention further relates to methods and compositions for detecting, diagnosing, prognosing, preventing, treating or ameliorating autoimmune diseases or disorder, such as systemic lupus erythematosus Rheumatoid arthritis, and Sjögren's syndrome, or cancers of the immune system, particularly B cell cancers such as non-Hodgkin's lymphoma and multiple myeloma, comprising administering to an animal, preferably a human, an effective amount of one or more antibodies or fragments or variants thereof, or related molecules, that immunospecifically bind to APRIL.