Foreign body response is an immune reaction to a foreign body implanted into the human body. This foreign body can be a knee joint replacement prosthesis, a prosthetic heart valve, or a microdevice with cells to release therapeutic factors. The transplantation of a object triggers this biological reaction, which is the natural protection mechanism of the host.

When a foreign object is implanted, the injury to the tissue initiates the inflammatory response. Proteins in the body adhere to the foreign object and this new protein layer promotes the invasion of immune cells to the location of the foreign body and their adhesion to its surface. Immune cells called macrophages fuse together and form foreign body giant cells. The cells release proteins that boost the inflammatory response. The cells then deposit a matrix that eventually forms a fibrotic capsule to encase foreign body. This fibrotic encapsulation can impair the functionality of the implanted device and lead to its failure.

The material of the implanted device therefore has an important impact on the immune response. Several strategies, from using biocompatible materials to coating materials with chemicals that limit fibrosis, have been developed to prevent this reaction. These technologies have enabled the long-term function of transplanted devices.