When was sublingual immunotherapy (under-the-tongue serum drops) developed?

Sublingual immunotherapy was developed over 60 years ago. It is widely used throughout the world and is especially popular in Central Europe where it accounts for more than 45 percent of allergy treatment.1

1Cox L, Jacobsen L. “Comparison of allergen immunotherapy practice patterns in the United States and Europe.” Annals of Allergy, Asthma and Immunology Dec 2009