Fcg receptors play a crucial role in immune defense by linking antibody-antigen complexes with cellular effector functions. The binding of IgG immune-complexes to Fcg receptors can either initiate inflammatory, cytolytic, and phagocytic activities of immune effector cells or down-regulate immune responses. So that we can understand the mechanism of antibody-mediated immune responses, the relationship between the structures and the functions of Fcg receptors needs to be elucidated. This can be accomplished by investigating the properties of Fcg receptor-IgG binding. To characterize the properties, based on the crystal structures of Fcg receptor-IgG complexes, molecular dynamics simulations and MM/PB(GB)SA will be implemented. Besides, X-ray crystallography will be used to solve the structures of FcgRIII isotypes and mutants. Besides Fcg receptors, integrins will also be studied because they are physiologically important adhesion molecules that mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. According to experimental data, researchers suggest that integrins have multiple conformations and must be activated in order to function. Many believe that the activation of integrins is accomplished by their conformational change. I propose to investigate the conformational change of integrins by normal mode analysis, implicit water models, and coarse grain models in order to explore the mechanism of integrin activation.