The present invention provides molecules that mimic antigenic determinants of the integral transmembrane protein claudin 18.2 (CLDN18.2). These molecules compete with CLDN18.2 for binding to a CLDN18.2 binding domain, e.g. a CLDN18.2 binding domain of an antibody, and are capable of detecting antibodies against CLDN18.2. The mimotopes of the invention may be used to generate or inhibit immune responses in animals and preferably humans. Furthermore, they can be used for purposes of detecting agents comprising a CLDN18.2 binding domain in biological samples as well as for purifying agents comprising a CLDN18.2 binding domain.