Illustration of the antigen binding fragment (Fab) of a cross-reactive neutralising antibody (centre) that targets the CD4 binding site of the HIV-1 (human immunodeficiency virus-1, top left and bottom right) gp120 protein. Antibodies are proteins of the immune system that bind to specific antigens, such as a viral protein. Cross-reactive antibodies are able to recognise antigens across different subtypes of a virus. Neutralising antibodies eliminate the pathogen themselves, rather than marking the pathogen for destruction by other immune cells. 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.