CD4 and infection: The last three equations of the model system (1), equations (7)–(9), describe the evolution in time of the population of ruminant immune response cells at a site of infection in the gut which are specific naive CD4 T + cells (T0) and the two subsets of the MAP specific immune response, Th1 (T1) and Th2 (T2) cells (see [12] and reference therein).