Accurate diagnosis of triggers or causative allergens is essential for appropriate risk assessment, providing correct advice to patients with allergy and their caregivers and personalized treatment. However, allergens have never been represented in the World Health Organization International Classification of Diseases (ICD). In this article, we present the process of selection of allergens to better fit the ICD, 11th Revision (ICD-11) structure and the outcomes of this process. The Logical Observation Identifiers Names and Codes database, containing 1444 allergens, was used as the basis for the selection process. Two independent experts were responsible for the first selection of the allergens according to specific technical criteria. The second step of the selection process was based on real-life relevance of the allergens according to the frequency of requests regarding each of them. We selected 1109 allergens (76.8%) from all 1444 present in the Logical Observation Identifiers Names and Codes database, with considerable agreement between experts (Cohen κ = 8.6). After assessment of real-life data, 297 additional relevant allergens worldwide were selected and grouped as plants (36.4%), drugs (32.6%), animal proteins (21%), mold and other microorganisms (1.5%), occupational allergens (0.4%), and miscellaneous allergens (0.5%). The stepwise approach allowed us to select the most relevant allergens in practice, which is the first step to building a classification of allergens for the WHO ICD-11. Aligned with the achievement in the construction of the pioneer section addressed to the allergic and hypersensitivity conditions in the ICD-11, the introduction of a classification for allergens can be considered timely and much needed in clinical practice.