Nonspecific protein-protein interactions of a monoclonal antibody were quantified experimentally using light scattering from low to high protein concentrations (c(2)) and compared with prior work for a different antibody that yielded qualitatively different behavior. The c(2) dependence of the excess Rayleigh ratio (R-ex) provided the osmotic second virial coefficient (B-22) at low c(2) and the static structure factor (S-q=0) at high c(2), as a function of solution pH, total ionic strength (TIS), and sucrose concentration. Net repulsive interactions were observed at pH 5, with weaker repulsions at higher TIS. Conversely, attractive electrostatic interactions were observed at pH 6.5, with weaker attractions at higher TIS. Refined coarse-grained models were used to fit model parameters using experimental B-22 versus TIS data. The parameters were used to predict high-c(2) R-ex values via Monte Carlo simulations and separately with Mayer-sampling calculations of higher-order virial coefficients. For both methods, predictions for repulsive to mildly attractive conditions were quantitatively accurate. However, only qualitatively accurate predictions were practical for strongly attractive conditions. An alternative, higher resolution model was used to show semiquantitatively and quantitatively accurate predictions of strong electrostatic attractions at low c(2) and low ionic strength.