A chemical released by various cells, in particular white blood cells to enhance the cellular immune response to an infectious agent. (The generic terms for such chemicals is cytokine). Animals that are, or have been infected with MAP have cells circulating in their blood that have been “trained” to recognize the antigens of this bacterium and respond by releasing significant amounts of gamma interferon. This can be tested for in the laboratory by mixing a blood sample, collected to preserve live leukocytes (white blood cells), with antigens of MAP. After overnight incubation the blood is then tested for gamma interferon using a commercially available kit. This test for cellular immunity to MAP shows considerable promise as a method for detecting the infection in young animals. Similar gamma interferon assays are under development for animals other than cattle.