A method of modulating immunity in a mammal is provided by modulating Galectin 9 activity in the mammal. Promoting or enhancing immunity may be effected by activating or stimulating Galectin 9 activity in the mammal such as by administering a Galectin 9 agonist to the mammal. The agonist may be multimeric soluble PD L2 or an agonist antibody that binds Galectin 9. Suppressing or preventing immunity may be achieved by inhibiting or blocking Galectin 9 activity in the mammal such as by administering an antagonist antibody or antibody fragment that binds Galectin 9 or that prevents or inhibits PD L2 multimerisation and/or binding to Galectin 9. The aforementioned methods may be suitable for preventing or treating a disease disorder or condition responsive to modulation of Galectin 9 activity. Also provided is a method of designing screening engineering or otherwise producing a Galectin 9 agonist inhibitor or antagonist that may be useful for modulating immunity by modulating Galectin 9 activity.
