Tumour necrosis factor-alpha (TNF-alpha), computer model. This molecule is composed almost entirely of anti-parallel beta-sheets (blue). This protein generally exists as a trimer (a molecule consisting of 3 identical smaller molecules, seen here). It is released by white blood cells, mainly macrophages, during inflammatory immune responses, and acts as a signalling molecule. Its release is triggered by injury or bacterial endotoxins. One of its actions is to kill tumour cells, hence its name. TNF-alpha is also involved in a number of inflammatory diseases, including rheumatoid arthritis, psoriasis and Crohn's disease.