Intravenous immunoglobulins (IVIg) are therapeutic preparations of normal human IgG obtained from pools of more than 1000 healthy blood donors. They are currently used in the treatment of a wide range of auto-immune diseases, whether associated with auto-antibodies or auto-reactive T lymphocytes, as well as in the treatment of systemic inflammatory diseases. Several mechanisms of action have been identified during the last 20 years, including: (i) modulation of Fc receptors expression on leukocytes and endothelial cells; (ii) interaction with complement proteins; (iii) modulation of cytokines and chemokines synthesis and release; (iv) modulation of cell proliferation and apoptosis; (v) remyelinisation; (vi) neutralisation of circulating autoantibodies; (vii) selection of repertoires of B and T lymphocytes; (viii) interaction with other cell-surface molecules on lymphocytes and monocytes; (ix) corticosteroid sparing. These mechanisms of action are multiple and often intricate. However, they are still little known and further investigations are warranted.