Tumor necrosis factor b (TNFb), also known as lymphotoxin, is a pleiotropic cytokine. TNFa, also known as cachectin, is a smaller cytokine that binds to the same receptors, producing a vast array of effects similar to those of TNFb. TNFb and TNFa share 30% amino acid homology and have similar biological activities. TNFb is produced by activated lymphocytes, including CD4(+) T helper cell type 1 lymphocytes, CD8(+) lymphocytes and certain B lymphoblastoid cell lines. TNFa is produced by several different cell types, including lymphocytes, neutrophils and macrophages. TNFa and TNFb can modulate many immune and inflammatory functions, while having the ability to inhibit tumor growth. Target tumor cells must express TNF receptors 1 and 2 to be killed, with the p55 receptor mediating the cytotoxic response.