A large number of allergenic proteins have now their complete cDNA sequences determined and in some cases also the 3D structures. It turned out that most allergens could be grouped into a small number of structural protein families, regardless of their biological source. Structural similarity among proteins from diverse sources is the molecular basis of allergic cross‐reactivity. The clinical relevance of immunoglobulin E (IgE) cross‐reactivity seems to be influenced by a number of factors including the immune response against the allergen, exposure and the allergen. As individuals are exposed to a variable number of allergenic sources bearing homologous molecules, the exact nature of the antigenic structure inducing the primary IgE immune response cannot be easily defined. In general, the ‘cross‐reactivity’ term should be limited to defined clinical manifestations showing reactivity to a source without previous exposure. ‘Co‐recognition’, including by definition ‘cross‐reactivity’, could be used to describe the large majority of the IgE reactivity where co‐exposure to a number of sources bearing homologous molecules do not allow unequivocal identification of the sensitizing molecule. The analysis of reactivity clusters in diagnosis allows the interpretation of the patient's reactivity profile as a result of the sensitization process, which often begins with exposure to a single allergenic molecule.

In late summer, pollen grains originating from Compositae weeds (e.g., mugwort, ragweed) are a major source of allergens worldwide. Here, we report the isolation of a cDNA clone coding for Art v 1, the major allergen of mugwort pollen. Sequence analysis showed that Art v 1 is a secreted allergen with an N-terminal cysteine-rich domain homologous to plant defensins and a C-terminal proline-rich region containing several (Ser/Ala)(Pro)2-4 repeats. Structural analysis showed that some of the proline residues in the C-terminal domain of Art v 1 are posttranslationally modified by hydroxylation and O-glycosylation. The O-glycans are composed of 3 galactoses and 9-16 arabinoses linked to a hydroxyproline and represent a new type of plant O-glycan. A 3-D structural model of Art v 1 was generated showing a characteristic "head and tail" structure. Evaluation of the antibody binding properties of natural and recombinant Art v 1 produced in Escherichia coli revealed the involvement of the defensin fold and posttranslational modifications in the formation of epitopes recognized by IgE antibodies from allergic patients. However, posttranslational modifications did not influence T-cell recognition. Thus, recombinant nonglycosylated Art v 1 is a good starting template for engineering hypoallergenic vaccines for weed-pollen therapy.

Ragweed and mugwort are important allergenic weeds belonging to the Asteraceae or Compositae plant family. Pollen of mugwort is one of the main causes of allergic reactions in late summer and autumn in Europe and affects about 10–14% of the patients suffering from pollinosis. Ragweed pollen represents the major source of allergenic protein in the United States, with a prevalence of about 50% in atopic individuals. In Europe, ragweed allergy is now rapidly increasing particularly in certain areas in France, Italy, Austria, Hungary, Croatia, and Bulgaria. Amb a 1 and Art v 1, the major allergens of ragweed and mugwort, respectively, are unrelated proteins. Amb a 1 is an acidic 38-kDa nonglycosylated protein. The natural protein undergoes proteolysis during purification and is cleaved into a 26-kDa alpha chain, which associates noncovalently with the beta chain of 12 kDa. The two-chain form seems to be immunologically indistinguishable from the full-length molecule. Art v 1 is a basic glycoprotein comprising two domains: an N-terminal cysteine-rich, defensin-like domain and a C-terminal proline/hydroxyproline-rich module. The proline/hydroxyproline-rich domain was recently shown to contain two types of glycosylation: (1) a large hydroxyproline-linked arabinogalactan composed of a short β1,6-galactan core substituted by a variable number (5–28) of α-arabinofuranose residues forming branched side chains with 5-, 2,5-, 3,5-, and 2,3,5-substituted arabinoses, and (2) single and adjacent β-arabinofuranoses linked to hydroxyproline. As described for other pollen, ragweed and mugwort pollen also contain the pan-allergen profilin and calcium-binding proteins, which are responsible for extensive cross-reactivity among pollen-sensitized patients.

Pollinosis patients often display adverse reactions upon the ingestion of plant‐derived foods as a result of immunoglobulin E (IgE) cross‐reactive structures shared by pollen and food allergen sources. The symptoms of such pollen‐food syndromes (PFS) or class 2 food allergies range from local oral allergy syndrome to severe systemic anaphylaxis. Two clinical syndromes, the celery‐mugwort‐spice syndrome and the mugwort‐mustard‐allergy syndrome have been described in association with weed pollinosis. However, other associations between weed pollinosis and hypersensitivity to certain kinds of food have also been observed, like the mugwort–peach, the ragweed–melon–banana, the plantain–melon, the pellitory–pistachio, the goosefoot–fruit, the Russian thistle–saffron, and the hop–celery association. The number of allergen sources involved, the allergens, and influencing factors including geography, diet, and food preparation contribute to the high clinical complexity of PFS. So far, known causative cross‐reactive allergens include profilins, lipid transfer proteins, and high‐molecular weight allergens and/or glycoallergens. The current usage of nonstandardized allergen extracts poses additional problems for both diagnosis and therapy of PFS patients. Further identification and characterization of involved allergens is inescapable for better understanding of PFS and vaccine development. Panels of recombinant allergens and/or hypo‐allergens are promising tools to improve both PFS diagnostics and therapy.

Art v 1, the major allergen of mugwort (Artemisia vulgaris) pollen contains galactose and arabinose. As the sera of some allergic patients react with natural but not with recombinant Art v 1 produced in bacteria, the glycosylation of Art v 1 may play a role in IgE binding and human allergic reactions. Chemical and enzymatic degradation, mass spectrometry, and 800 MHz 1 H and 13 C nuclear magnetic resonance spectroscopy indicated the proline-rich domain to be glycosylated in two ways. We found a large hydroxyproline-linked arabinogalactan composed of a short ␤1,6-galactan core, which is substituted by a variable number (5-28) of ␣-arabinofuranose residues, which form branched side chains with 5-, 2,5-, 3,5-, and 2,3,5-substituted arabinoses. Thus, the design of the Art v 1 polysaccharide differs from that of the well known type II arabinogalactans, and we suggest it be named type III arabinogalactan. The other type of glycosylation was formed by single (but adjacent) ␤-arabinofuranoses linked to hydroxyproline. In contrast to the arabinosylation of Ser-Hyp 4 motifs in other hydroxyproline-rich glycoproteins, such as extensins or solanaceous lectins, no oligo-arabinosides were found in Art v 1. Art v 1 and parts thereof produced by alkaline degradation, chemical deglycosylation, proteolytic degradation, and/or digestion with ␣-arabinofuranosidase were used in enzyme-linked immunosorbent assay and immunoblot experiments with rabbit serum and with the sera of patients. Although we could not observe antibody binding by the polysaccharide, the single hydroxyproline-linked ␤-arabinose residues appeared to react with the antibodies. Mono-␤-arabinosylated hydroxyproline residues thus constitute a new, potentially cross-reactive, carbohydrate determinant in plant proteins.