Whatever substance is injected within human tissues, it will start a series of reactions that must be known. These reactions are part of the normal healing process, with the aim of removing or isolating the foreign body from the host tissue. They follow three phases: recognition of the foreign body, removal or isolation, and a final healing phase. But these reactions can be either reduced or enhanced by the structure of the injected filler. Therefore it seems important to well understand these mechanisms to choose, if not the best, then the least harmful product available.

FOREIGN BODY REACTION: THE INFLAMMATORY PROCESS

Phase 1: The Foreign Body Must Be Identified

Identification is the role of the monocytes from the bloodstream, activated into macrophages by factors released through the wound, even a minimal puncture, by platelets coming into contact with the extracellular matrix (ECM).

Adhesion

Macrophages must adhere to the foreign body. This adhesion is the most important aspect of the cellular interaction. It is mediated by adsorption, the deposition of glycoproteins from the ECM and/or plasma on the surface of the foreign body. The more the proteins cover the surface, the more cell adhesion, spreading, and proliferation will be efficient.

Recognition

These deposited proteins start a specific recognition by monocyte/macrophage surface cell receptors. There is also a nonspecific interaction between cell surface molecules (oligosaccharides), the adsorbed proteins, and the implant surface.

Phase 2: The Foreign Body Must Be Removed

Removal is accomplished through phagocytosis from:

1. monocytes/macrophages

2. neutrophils/polymorphonuclears

3. keratinocytes, if the implant is superficial (papillary dermis).