Antibody-dependent enhancement (ADE) occurs when non-neutralising antiviral proteins facilitate virus entry into host cells, leading to increased infectivity in the cells. Some cells do not have the usual receptors on their surfaces that viruses use to gain entry. The antiviral proteins (i.e., the antibodies) bind to antibody Fc receptors that some of these cells have in the plasma membrane. The viruses bind to the antigen binding site at the other end of the antibody. ADE is common in cells cultured in the laboratory, but rarely occurs in vivo except for dengue virus. This virus can use this mechanism to infect human macrophages, causing a normally mild viral infection to become life-threatening.