The functional significance of carbohydrates in biological systems is quite diverse. For example, carbohydrates cover the surface of some bacteria, such as Haemophilus influenzae. Carbohydrates in the outer lipid coat may aid in disguising these bacteria by mimicking substances naturally present in humans. They are also key components of cartilage, where they form polymeric chains of negatively charged monosaccharides. The negative charges provide electrostatic repulsion when the cartilage undergoes compression. Carbohydrates also are present on the surface of many proteins. Their presence is sometimes necessary to assist in protein folding or receptor binding. The protein that covers the surface of the human immunodeficiency virus (HIV), gp120, is heavily glycosylated; these carbohydrates are believed to shield the protein from immune attack.