This invention provides a process for producing major histocompatibility antigen class II protein (hereinafter referred to as “MHC class II” for short) which occurs on the surfaces of antigen-presenting cells and the like, and MHC class II-bound materials in which MHC class II, &agr; and/or &bgr; subunit of MHC class II, or a part thereof is bound to a carrier such as beads, fibers and hollow fibers via covalent bond, as well as a module for removing superantigen using the same. This invention also provides a method for detecting or quantifying superantigens using MHC class II or a part thereof having an affinity to the superantigens, as well as an assay kit therefor.