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Background: Asthma, rhinitis, and eczema are common. Considerable multimorbidity exists involving all three diseases, which cannot be explained by chance. Allergic sensitisation increases the risk of multimorbidity, though not much is known about other factors impacting their co-occurrence.

Aim: The overall aim of this thesis was to investigate factors associated with allergic multimorbidity. 

Methods and results: We have investigated allergic multimorbidity in terms of background and demographic factors, symptom burden, lung function, allergic sensitisation, and inflammatory markers: total immunoglobulin E (IgE), C-reactive protein (CRP), periostin, fractional exhaled nitric oxide (FENO), alveolar nitric oxide (CANO), blood eosinophil counts, eosinophil activation markers (eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN)).  

The study populations were mainly middle-aged adults, except for paper III, which comprised children and young adults. Paper I included 216 individuals from the European Community Respiratory Health Survey (ECRHS II). Paper II was based on 437 individuals from the Swedish part of the Global Allergy and Asthma European Network (GA2LEN). Paper III involved 411 participants from the Minimally Invasive Diagnostics for Asthma and Allergic Diseases Study (MIDAS). Paper IV was based on 255 individuals who were followed for 10 years from ECRHS II to ECRHS III. 

The main findings were that individuals with allergic multimorbidity were more likely to be polysensitised to allergens and had higher levels of total IgE compared with those with only one disease. Both factors, as well as higher FENO and ECP at baseline, were associated with persistent allergic disease over time. Multimorbidity was associated with higher levels of FENO in subjects with asthma aged under 18 years and EDN. Those with multimorbidity reported more asthma and allergy symptoms, had heredity, especially maternal heredity. We found both higher and lower lung function among those with more than one allergic disease, which underscores the heterogeneity of asthma as a disease. 

Conclusion: We found allergic multimorbidity to be associated with polysensitisation, higher levels of type 2 inflammation, and higher symptom burden compared with those with only one allergic disease. Highlighting the importance of multimodal management when striving to decrease the symptom burden and socioeconomic cost of allergic multimorbidity.