This section includes isotype controls for different applications. Primary antibodies that are complete (i.e. still contain the Fc-Region) can bind to cell surfaces on human leukocytes and can be the cause of background signals. Isotype controls that are directed against an antigen that is not present in the analyzed cell population represent this background signal and can be used as a negative control for better measuring the specificity of the primary antibody. The isotype control should match the isotype of the primary antibody in the heavy chain (IgA, IgG, IgD, IgE or IgM) and should also consist of the same light chain (Kappa or Lambda). If the antibody is directly conjugated the fluorescent dye should be identical.