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.
