Immunotherapy is a safe and effective method of desensitisation for seasonal and perennial allergies.  It can help hundreds of people in the UK who suffer form hay fever and allergic asthma. It requires at least four months before the season to be effective the same year.  For grass pollen it usually time to start from November till January to give maximum benefit by June.

 https://www.allergycliniclondon.co.uk/immunotherapy/