In addition, passive transfer studies in a relevant monkey model suggest that Fc receptor (FcR) binding capacity of a protective antibody makes a substantial contribution to the antibody-mediated protection [41] . Antibody effector functions that mediate complement activation and FcR engagement on macrophages, dendritic cells, natural killer cells, and other cell types need to be evaluated to determine their relevance to HIV-1 vaccines. Assays that measure these antiviral antibodies should be standardized and used to assess biologic relevance in passive protection experiments in animal models using antibodies that exhibit the different effector functions in vitro.