Corynebacterium pseudotuberculosis is a facultative intracellular pathogen with pleomorphic forms. While two forms of the bacteria exist, in our study, we are most concerned with the equi biovar, the strain that affects horses and cattle. The major secreted exotoxin is phospholipase D (PLD), which catalyzes the hydrolysis of ester bonds in phospholipids. Management of the disease is highly challenging, as antibiotics are often ineffective since bacteria stay protected inside abscesses within thick enclosing capsules. This leads to the overarching goal of the study: the development of a vaccine against C. pseudotuberculosis . Since the disease can manifest either internally or externally, further understanding of the roles of the antibody and cell-mediated responses in determining the form of disease is crucial. Important stepping-stones for success include the establishment of the roles TH1 and TH2 lymphocytes in response to the bacteria and the comparison of antibody responses in different manifestations of the disease. The Enzyme Linked Immuno-Sorbent Assay (ELISA) developed specifically for the detection of PLD specific antibodies was utilized to study the antibody response, specifically to determine the linkage between levels of IgE and IgA to the different manifestations of the disease. The cell-mediated response is studied by determining which cytokines are produced by T helper cells in response to PLD. Cells are stimulated in culture and RNA isolated for determining cytokine level by TaqMan real-time RT-PCR. By determining the types of cytokine produced, a cell-mediated response can be identified as aTH1 or TH2 response.