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All gels act as foreign bodies. Host response ranges from a few macrophages to an intense foreign-body reaction with fibrosis, depending on gel type. For polymer gels the filling effect stems from their volume. For combination gels it stems from the intended host foreign-body reaction to the microparticles.This review explains how host tissue reacts with different gel types and how adverse reactions differ depending on gel type.This study is based on observations during the past 6 1/2 years from experimental/clinical studies and biopsies from adverse reactions.Hydrophilic polymer gels constantly exchange water with their host tissue with minimal fibrosis. Degradable gels are encircled and degraded by macrophages; polyacrylamide gel is traversed by macrophages, replaced by fibrous, vessel-bearing strands. All other gels provoke a foreign-body reaction with ensuing fibrosis. Adverse reactions include bacterial infection for the hydrophilic gels and a fibrous granuloma for all other gel types. It is hypothesized that bacteria are maintained within a biofilm (a film comprising bacteria, their nutrients, and their waste products) giving rise to a low-grade infection.Infectious nodules must be treated with antibiotics. Granulomas must be treated with a combination of both steroids and antibiotics or excision.