Plant cell walls undergo remodeling during growth and development and are the first target of many invading pathogens. Acidic pectin (homogalacturonans) binds calcium and forms chain dimers called egg boxes and even multimers at higher calcium ion concentrations. Chitosan, the deacetylated form of chitin produced by fungi when invading plant tissues, is a cationic polymer that can interact with negatively charged pectin. The interaction between chitosan oligomers (COS) and pectic egg boxes was investigated using 2F4, a monoclonal antibody specific for calcium-associated dimers of pectin. Depending on the size of the pectic molecules, the COS to pectin ratio, the degree of polymerization and the degree of acetylation of COS in the mixture, the calcium-induced egg box conformation of oligogalacturonides (OGA) was strongly stabilized or destroyed. The biological activity of COS-stabilized egg boxes was assayed on Arabidopsis cell suspensions. COS-OGA egg boxes strongly enhanced extracellular alkalinization and decreased potassium fluxes compared to control COS and OGA alone. Furthermore, OGA rescued Arabidopsis from cell death induced by higher concentrations of deacetylated COS. The stabilized COS-OGA egg boxes could constitute a combined emergency signal that informs plant cells on both cell wall degradation and pathogen presence, triggering a much stronger response than individual components alone.