g202 Maize, Corn

Maize (Zea mays), also known as corn, is an annual grass cultivated globally and one of the most important cereal crops worldwide. Maize pollen can elicit respiratory symptoms in sensitized individuals living near growing areas, or during occupational exposure. A number of allergenic molecules within Z. mays pollen contribute to variable cross-reactivity with their homologues in other monocot and dicot plants and grasses, including foods and latex.

Nature

Zea mays is an annual grass composed of four subspecies: domesticated maize (ssp. mays), a wild progenitor ssp. parviglumis, and the wild taxa ssp. mexicana and ssp. huehuetenangensis (1). In its center of origin in Central America, cultivated maize is found in close proximity to all of the wild Zea taxa, and may exchange genes with its wild relatives (1).

Following rapid geographical dispersion over the last few centuries, maize is now cultivated globally as one of the most important cereal crops worldwide and is a source of income for many populations in developing countries (2, 3). During domestication, maize has adapted to become a day-neutral plant that can flower at higher latiude and in longer daylight conditions than its historical ancestors (4).

Maize plants can exceed 2 m in height, with inflorescences of male flowers called tassels that release pollen under suitably warm and dry conditions (2). Maize pollen is anemophilous (dispersed by wind) and most pollen falls within 50–70 meters of the plant due to its weight (150–500 ng) (2). However, rapid dehydration of the pollen within a few hours of exposure to the air can potentially increase transport distances by inducing changes in physical shape which increase specific gravity and decrease settling speed (5).

Taxonomy

Pollen produced by Z. mays may induce hay fever, asthma and conjunctivitis in sensitized individuals.

Exposure to maize pollen can elicit symptoms in seasonal hay fever patients (7), and positive skin prick test (SPT) results have confirmed Z. mays pollen as an important allergen among sensitized individuals (8-10). The weight of the pollen grain means that allergic symptoms are most likely to be elicited in upper respiratory airways, and rural populations near growing fields are more likely to be exposed to maize pollen than urban populations (2).

However, the degree to which exposure to maize pollen affects sensitization is unclear. A population study in France reported a positive correlation between a high rate of exposure to maize pollen and sensitization, with higher rates of sensitization also seen in adults and children with atopic disease compared to those individuals with asymptomatic atopy (11). On the other hand, a study of individuals working directly with maize pollen in Germany found no association between duration of exposure and the frequency of sensitization (2). In this study, eight subjects exposed to maize pollen during their work in a biological research department reported suffering for several weeks from rhinitis (n=5), conjunctivitis (n=4), urticaria (n=4) and shortness of breath (n=2); most of these workers developed allergic symptoms between 1 to 7 months after the onset of maize pollination (2). All symptomatic patients in this study had specific IgE antibodies against maize pollen, and the allergenic components were identified as Zea m 1 and Zea m 13 (2).

Other studies have also suggested that occupational exposure may be a high-risk activity for the development of sensitization to maize pollen (12). A case report described a man aged 55 years with a 16-year history of recurrent episodes of rhinoconjunctivitis and asthma during maize pollen season (12). Despite working in an area with abundant maize, fruit trees and vegetables, this individual showed negative SPTs to a wide range of pollens and other aeroallergens, with the exception of maize pollen (specific IgE 4.28 kU/l) (12).

Of note, while agricultural workers exposed to the dust of stored maize corn have also reported respiratory symptoms, it is important to distinguish that these symptoms may actually have been caused by maize dust contaminants such as molds or thermophilic actinomycetes (13).

In Arizona (USA), six of nine atopic patients had positive SPT to corn pollen used in Navajo ceremonials, four of whom additionally reported symptoms from oral corn pollen including various combinations of oral and ear itching, sneezing, cough and wheezing (14). An earlier publication by the same author reported a 4-year old Navajo child who developed sudden onset cough, sneezing and mild respiratory distress approximately 15 minutes after corn pollen was put in her mouth for ceremonial purposes (15). In this patient, symptoms resolved spontaneously within 30 minutes, and SPT revealed a positive reaction to corn pollen, but not to corn food (15). 

Allergenic molecules

Allergens of Zea mays pollen(16) (Allergome 2021)

The major allergens of Zea mays pollen are Zea m 1 and Zea m 13 (17)

Cross-reactivity

A certain degree of cross-reactivity among members of the Poaceae family may be expected as most species of grass and maize pollen contain at least the Group 1 and Group 13 grass allergens (2, 17, 18). However, low degrees of cross-reactivity between grass and maize pollens have been observed in studies utilizing RAST inhibition tests (19-21), and peptide mapping analysis of Group 1 allergens in eight different clinically important grass pollens found no IgE-binding similarity between maize and other grasses (22).  

Cross-reactivity is more likely between maize and structurally similar members within the subfamily Panicoideae (e.g. Bahia grass and Johnson grass) than between maize and European native grasses of the separate subfamily Pooideae (e.g. timothy grass, orchard grass, rye grass or sweet vernal grass) (2, 21, 23). Indeed, anecdotal clinical experience has described patients with seasonal hay fever including asthmatics who showed positive SPTs to maize pollen extracts but not to grasses, while other patients showed positive SPTs to grass pollen but not to maize pollen (7). An Australian study reported that Pas n 1 from Bahia grass pollen (Paspalum notatum) was more closely related to maize Zea m 1 (85% identity) than to rye grass Lol p 1 (64% identity) or timothy grass Phl p 1 (64% identity) (24). In Central Europe, a sequencing study of the maize allergens Zea m 1 and Zea m 13 identified 72% and 70% sequence identities, respectively, to the corresponding Phl p 1 and Phl p 13 allergens of timothy grass pollen (Phleum pratense) (17) (17).

Profilins, considered a pan-allergen as they are present in all eukaryotic cells, may reach a prevalence of 60% in allergic individuals in areas with high pollen exposure, and induce severe food-allergic reactions even at very low concentrations (25). High homology has been demonstrated between profilin from maize (ZmPRO3) and pollen-specific profilin from rice (Oryza sativa, 89%) (26) and tomato (Lycopersicon esculentum, 77%) (27).

In one study, more than half of 56 maize pollen-sensitized children with hay fever symptoms were also sensitized to maize seed allergens (28), which may be due to Zea m 13 and homologous proteins which are present in both maize pollen and seed (2).

The thioredoxin Zea m 25 shares 74% sequence identity and distinct IgE cross-reactivity with its wheat homologue Tri a 25 (Triticum aestivum), which may be of relevance for patients occupationally exposed to these inhalants such as bakers (29). Levels of other allergens in maize pollen may be low, as two studies failed to detect substantial amounts of immunologically reactive Group 5-related allergens in maize (Z. mays) and Bermuda grass (Cynodon dactylon) pollen extracts (19, 30); Flicker et al. (2000) additionally failed to detect substantial amounts of Group 2 or Group 6-related allergens in these species (30).

Last reviewed:April 2022