Neutralization of dengue virus (DENV) infection occurs when DENV-specific antibodies prevent binding between the virion and the cell surface receptor, or endosome membrane fusion at a later stage. In antibody-dependent enhancement, DENV-specific antibody levels are too low to cover all the virion binding sites necessary for cell entry. Virus entry is then facilitated by attachment of virion- antibody complexes to Fc receptors found on the surface of immune cells including B lymphocytes, natural killer cells, macrophages, neutrophils and mast cells.
Reference: Alan L. Rothman. Immunity to dengue virus: a tale of original antigenic sin and tropical cytokine storms. Nature Reviews
Immunology 2011; 11: 532-43. Available at http://www.nature.com/nri/journal/v11/n8/fig_tab/nri3014_F1.html. 
Antibody-Dependent Enhancement (ADE)