Exposure of blood to hemodialysis membranes results in numerous interactions between the blood elements and the membrane. Transformation and adsorption of plasma proteins (such as complement) and activation of blood cells (such as neutrophils and monocytes) have been studied most extensively by nephrologists in recent years. There is no consensus on the definition of biocompatibility for dialyzer membranes. An operational definition of biocompatibility is the lack of any perturbation of blood constituents. According to this "inert surface" definition, a membrane (for example, one that adsorbs beta 2-microglobulin) can be considered as bioincompatible and yet desirable. Because of the multitude of blood-membrane interactions that may occur during hemodialysis, multiple criteria for biocompatibility needs to be applied in the classification of membranes. A certain bioincompatible phenomenon can be further classified as beneficial or deleterious depending on its biological effects as well as its acute and chronic impacts on the dialysis patient.