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Like other enveloped viruses, HIV exits its host cell enshrouded in the cell’s membrane, which contains membrane molecules such as the human leukocyte antigens (HLA). The HLA proteins act as a set of cell identification marks: every person expresses a slightly different HLA set. These molecules differentiate one person from another and allow the immune system to detect foreign invaders, and to reject tissue from other people or animals. Interestingly, each HIV particle has many more human HLA on its envelope surface than it has its own gp120 viral coat proteins, which the virus needs to bind to CD4 and CCR5 or CXCR4 on the lymphocyte surface in order to enter cells.