Allergenic molecules
The World Health Organization and International Union of Immunological Societies’ (WHO/IUIS) Allergen Nomenclature Subcommittee have registered three allergenic components, i.e., Car i 1 (2S albumin), Car i 2 (vicilin like protein) and Car i 4 (legumin like protein) for pecan nut. (19, 20).
| Name of allergen |
Isoallergen |
Biochemical name |
Molecular weight (kDa) |
Allergenicity |
|---|
| Car i 1 |
Car i 1.0101 |
2S albumin |
16 |
Major allergen.
IgE binding was found in 79% of 28 sera and 42% out of 12 sera of pecan allergic patients (19, 21).
88% sequence identity between Car i 1 and walnut allergen Jug r 1 (22).
|
| Car i 2 |
Car i 2.0101 |
Vicilin-like protein |
55 |
IgE binding was found in ~30% of 27 sera and 24% of 25 sera of pecan allergic patients (17, 19).
92% sequence identity of Car i 2 with walnut allergen Jug r 2 and 44% with Jug r 6 (22).
|
| Car i 4 |
Car i 4.0101 |
Legumin seed storage protein |
Subunit of hexameric protein:55.4 kDa |
Major allergen.
IgE binding was found in 57% of 28 sera of pecan allergic patients (19).
95% sequence identity between Car i 4 and walnut allergen Jug r 4 (22).
|
| Name of allergen |
Isoallergen |
Biochemical name |
Molecular weight (kDa) |
Allergenicity |
|---|
kDa: kilodaltons, IgE: Immunoglobulin E
Evidence has revealed that dietary lipids coexisting in pecan nuts may serve as adjuvants for sensitization to pecan allergens (Car i 1, Car i 2 and Car i 4) by demonstrating coordinated accumulation of allergens and lipids during embryological development of pecan nut (23).
Biomarkers of severity
The major allergens Car i 1 and Car i 4 were found to be very stable post blanching for 10 mins, roasting at 148° C for 30 min/172° C for 12 min and autoclaving for about 5 mins. This suggests the presence of heat-stable allergens in pecans (24). Further, Car i 1 (2S albumin) was found to be very stable upon pepsin digestion and able to bind IgE immunologically in a similar way as undigested protein (21).
Cross-reactivity
In the Pronuts study, walnut and pecan were found as highly correlated nut allergies, where 97% of pecan allergic individuals were found to be allergic to walnuts, while only 75% of walnut allergic children were allergic to pecans (10). The NUT-CRACKER and NUT-CRACKER-PS study was conducted in Israel to study coincidences of allergies among tree nuts and predict walnut-pecan allergy severity respectively. A total of 134 of 183 patients (47 from NUT-CRACKER and 87 from NUT-CRACKER-PS study) were found to be walnut allergic. Pecan allergy was co-existing in 79.9% of walnut allergic patients (3.6-27.3 years), while 100% of pecan allergic patients were allergic to walnut (12, 25). This suggests the co-allergy to be unidirectional. The high association between walnut and pecan allergies (spearman coefficient=0.96) could be attributed to high sequence identity among the pecan allergens (Car i 1, Car i 2 and Car i 4) with their counterparts in walnut (Jug r 1, Jug r 2, Jug r 4) respectively (19, 22). Pecan allergy was also found to be significantly correlated with hazelnut (Odd’s ratio [OR]=14.9, p<0.0001, Pearson correlation coefficient = 0.47) and macadamia nut (OR=15.2, p<0.0001, Pearson correlation coefficient=0.36), while it was found to be inversely related with peanut allergy (Pearson correlation coefficient = -0.25) (10). Further, 66.7% of peanut-allergic patients showed negative results to specific skin prick tests for pecan (26). In a single-center, retrospective analysis of serum specific IgE levels to coconut, 51% of co-sensitization was reported between pecan and coconut but were not found to be significantly associated (27). A study conducted in Finland to determine cross-reactivities between birch pollen and tree nuts reported that 21% of subjects sensitized to birch pollens were sensitized to pecan also (13).
