Pooled human plasma is processed by cold ethanol fractionation to produce purified immunoglobulin G antibodies for intravenous administration. Immunoglobulin A is an unwanted by-product since intravenous administration of immunoglobulin A-containing immunoglobulin G can cause life-threatening anaphylaxis in some people. The present invention is the aerosol administration, by metered dose inhaler or nebulizer, of by-product immunoglobulin A for the prevention or treatment of diseases including immunodeficiencies and infections. Antigen-specific monoclonal immunoglobulin A may be used. Immunoglobulin A from any of the aforementioned sources may then be coupled with recombinant J chain, and may then be additionally coupled with recombinant secretory component in order to render the immunoglobulin A more physiologically active. Immunoglobulin A, with or without J chain and secretory component, is then administered by aerosol inhalation.