In the recent past a great number of proteins causing type 1 allergic reactions in humans have been isolated and characterised. The main sources containing allergens are plants, mites, fungal spores and insects. Plant-derived allergens may either be taken in from the upper respiratory tract or they are present in a vast range of plant food causing food allergic reactions. Compared to the enormous amount of different plant proteins only a small number out of them are identified as a an allergen at present. Looking at the allergen encoding sequences, relationships by sequence similarity can be found quite frequently to a restricted number of plant protein families. Predominantly, these protein families are seed storage proteins, structural proteins and proteins involved in the defence-related system – pathogenesis-related proteins. In the following, a short overview of a number of pathogenesis-related protein families is presented in relation to the already known homologous plant allergens.