Enzyme-linked immunosorbent assays can be used to detect isotype-specific anti-influenza antibodies in biological samples to characterize the porcine immune response to influenza A virus. The isotype antibody assay is based on an indirect ELISA using whole influenza virus as antigen and detection antibodies directed against porcine IgG and IgA. Samples such as serum, nasal wash, and bronchoalveolar lavage fluid allow for evaluation of systemic, upper, and lower respiratory tract mucosal antibody responses, respectively. The isotype ELISA assay is performed in a 96-well format using anti-swine detection antibodies conjugated to an enzyme that catalyze a color change reaction. The optical density of the sample is measured using an automated plate reader. The assay is useful to characterize the IgG or the IgA response to challenge or vaccination against specific influenza virus isolates in different compartments of the immune system.