There are three types of foreign body reactions to plastic implants: 1) reactions due to physical characteristics of the implant, 2) reactions due directly to chemical properties of the implant, and 3) immune reactions. Responses which vary with the physical properties of the implant are epithelial encapsulation of the plastic, epithelial keratinization in cutaneous implants, thickening of the connective tissue fibrous capsule, formation of ground substance, and the presence of giant cells. Responses related directly to chemical toxicity of the plastic are epithelial hypertrophy (with mild irritants), inhibition of epithelial growth (with more toxic irritants), connective tissue inflammation, accumulation of a cellular glycoproteins, and vacuolization of host tissue. Finally, reactions due to infection or the presence of other antigens are characterized by inhibition of epithelial growth, invasion of epithelium by leukocytes, and proliferation of inflammatory tissue with a large population of plasma and other round cells.