An untreated allergy can make your immune system weak. It can make your body susceptible to viruses and bacteria.
References:
1.https://www.healthline.com/health/allergies#blood-test
2.https://my.clevelandclinic.org/health/diagnostics/21495-allergy-testing
3.https://www.allergy.org.au/patients/about-allergy/what-is-allergy