The present invention relates to compositions and methods for decreasing an immune response in an animal comprising administering to said animal an agent that binds a bioactive lipid and reduces the effective concentration of said bioactive lipid. Also provided are methods for treating diseases or conditions, including autoimmune disorders, which are characterized by an aberrant, excessive or undesired immune response. The methods of the invention utilize agents that bind bioactive lipids and are capable of decreasing the effective concentration of the bioactive lipid. In some embodiments, the agent is a monoclonal antibody that is reactive against sphingosine-1-phosphate (S1P) or lysophosphatidic acid (LPA).