Illustration of an immunoglobulin G b12 antibody against HIV-1 (human immunodeficiency virus-1). This antibody targets the CD4 binding site of the HIV-1 gp120 protein. Antibodies are proteins of the immune system that bind to specific antigens, such as a viral protein, and either mark the pathogen for destruction by other immune cells, or destroy it themselves. The HIV gp120 protein binds to CD4 receptors on human cells, facilitating the virus's entry to the cell. HIV is the cause of AIDS (acquired immune deficiency syndrome). The virus infects the immune system's T-lymphocytes, ultimately killing them, leading to a very weak immune system. HIV antibodies are a potential therapy for the treatment of HIV infection.