Methods of using one or more agonists or antagonists to modulate activity of the members of the TNF and TNFR families referred to as TALL-1, APRIL, TACI, and BCMA are provided. The methods include in vitro, in situ, and/or in vivo diagnosis and/or treatment of mammalian cells or pathological conditions associated with TALL-1, APRIL, TACI, or BCMA, using one or more agonist or antagonist molecules. The methods of treatment disclosed by the invention include methods of treating immune related diseases and cancer.