Illustration of a human immunodeficiency virus (HIV) particle, the cause of AIDS (acquired immune deficiency syndrome). The outer layer (seen here) is the envelope, a lipid bilayer membrane. Membrane proteins protrude from this surface. The larger brown membrane proteins are GP120, a glycoprotein that plays a vital role in the virus entering cells. Also seen are class I (yellow) and class II (orange) human leukocyte antigen (HLA) proteins. These are derived from the host cell's membrane, when the virus buds from the cell. Within the membrane is an icosahedral matrix protein shell that encloses the genetic core (not seen).