The development of noticeable signs may occur with stressful events such as calving, feed changes, relocation, etc. There is some data to suggest that subclinical stages may result in a decline in performance, especially in milk production in the last lactation before culling. M cells present the bacteria to macrophages and lymphocytes in Peyer s patches. The cellular immune system reacts to the invasion by recruiting more macrophages and lymphocytes to the site. Lymphocytes release a variety of cytokines, which enhance the bacterial killing ability of the macrophages.
Macrophages fuse into large cells and with lymphocytes infiltrate infected tissues in large numbers.
This leads to the granulomatous thickening of the intestine. In later phases of the immune response, bacteria escape macrophages into the intestinal lumen and animals begin to shed Map in feces. Eventually the bacteria also escape into the vascular system which stimulates antibody. This humoral immunity however does not play a significant role in controlling Map because of its intracellular localization. Figure 1 .