The association between alpha 1-antitrypsin (alpha 1-AT) deficiency and a number of immune mediated diseases including rheumatoid arthritis, anterior uveitis, systemic lupus erythematosus, and asthma suggests that alpha 1-AT may be important not only as an anti-inflammatory protein but also as an immune regulator. That the relationship between decreased amounts of this inhibitor and these diseases is causal is suggested by both some of its physical properties and evidence indicating it is able to modulate immune function. alpha 1-Antitrypsin has a high plasma concentration, very broad range of inhibitory activity and is an acute phase reactant. Among other things, it is able to modulate lymphocyte proliferation and cytotoxicity, and monocyte and neutrophil function. Additionally, some of these changes are demonstrable in vivo in patients with severe alpha 1-antitrypsin deficiency. This paper reviews the important physicochemical characteristics of this protein, the association of its presence in decreased amounts with immune disorders, and finally the important mechanism that may underlie this disease association.