Natalizumab is a humanised IgG4k monoclonal antibody produced in murine myeloma cells. Natalizumab contains human framework regions and the complementarity-determining regions of a murine antibody that binds to a4-integrin. Natalizumab is used to treat multiple sclerosis-MS. It is thought to help by preventing patient’s immune system from attacking the nerves in the brain and spinal cord. Natalizumab is also used to treat a bowel condition called Crohn's disease (CD). BioVision’s BioSim™ anti- Natalizumab ELISA kit is designed to detect antibody against Natalizumab with high specificity and sensitivity in serum and plasma samples. anti-Natalizumab ELISA is based on the sandwich ELISA principle. Controls and samples are incubated in the microtiter plate coated with the drug Natalizumab. After incubation, the wells are washed. Then, HRP conjugated probe is added and binds to Natalizumab antibodies captured by the drug Natalizumab on the surface of the wells. Following incubation wells are washed and the bound enzymatic activity is detected by addition of chromogen-substrate. Finally, the reaction is terminated with an acidic stop solution. The color developed is proportional to the amount of Natalizumab antibodies in the sample or controls. The results can be evaluated with using cut-off value.