Ustekinumab (trade name Stelara) is an active substance known as a monoclonal antibody. Monoclonal antibodies are proteins that recognise and bind specifically to certain proteins in the body. Ustekinumab belongs to a group of medicines known as immunosuppressants. These drugs have the effect of dampening down part of the immune system (the body’s own defence system), which may be overactive in some conditions.