In presence of an antigen in intestine, the organism can react of three different ways:

1. Immune exclusion: It consists of avoiding the entrance of the antigen in the organism. It is a non inflammatory process mediated by the specific immune response (IgA and IgM) and mechanisms such as the Ph, mucus or the peristaltism.

2. Immune elimination: It is a process which consists of eliminating the dangerous antigens by the specific immune response and above all the unspecific immune response, such as neutrophils, macrophages, mastocytes or others.

3. Immune regulation or oral tolerance: In this process, the organism does not generate immune response against an intestinal antigen, for example against food proteins.

Mechanisms of intestinal immunity

In the intestine, they can be found some specific immune mechanisms such as Peyer’s patches and intraepithelial lymphocytes.

1. Specific mechanisms: The lymphoid tissues associated with the intestine are continually exposed to antigens and they constitute the most part of the lymphocytes of the immune system (up to 70%). Many lymphocytes can be found together in the Peyer’s patches and in the mesenteric lymphatic nodules. These populations of lymphocytes are different from the functional and anatomic point of view of the peripheral populations and are capable of distinguishing between the pathogens and the beneficial compounds for the organism, such as food proteins and commensal bacteria.

1.1. Peyer’s patches: They constitute the immune inductor of the mucosa. They do not have villi and the migratory cells M which are concentrated in their surface are exposed to the protein antigens of the intestinal content (food and microorganisms). These antigens are processed and presented to the lymphocytes B which are located in germinal centres of the Peyer’s patches. The B lymphocytes circulate towards the bloodstream, mature by becoming plasmatic cells and go back to the gastrointestinal epithelium through lymphatic tissues associated to the intestinal mucosa to secrete A specific immune globulin against the antigen which has appeared. They also secrete IgM, IgG and IgE although in smaller amounts. The circulation of B lymphocytes is carried out in 1-2% of them per hour which allows a fast generation of immunity to the detected antigens.

1.2. Intraepithelial lymphocytes (LIE): The intraepithelial lymphocytes form a vast population of T lymphocytes with particular characteristics, located over the basal membrane of the intestinal epithelium, inserted with enterocytes. They represent the first immune competent cells to find antigens orally, and presumably they are capable of distinguishing between innocuous and pathogen antigens.

2. Unspecific immune system: In the intestine they also can be found mechanisms of the unspecific immune system such as neutrophils or macrophages, which are usually located in the own sheet and they are capable of destroying strange bodies. The system of the complement covers over 18 proteins. These proteins act in chain, I mean, that one activate the following. The system of the complement can be activated through 2 different ways. The first one, alternative way, is activated by certain microbial products or antigens. The other way, called classic, is activated by specific antibodies united to their antigens (immune complexes). The system of the complement destroys strange substances, directly or together with other components of the immune system.

Intestinal pathogens

Many intestinal pathogens such as the coccidian colonize the intestinal mucosa. This colonization causes an immune response in the animal, which can be specific or unspecific.

1. Unspecific. Immune elimination: macrophages have sensors which recognise the molecules of the surface of the pathogen microorganism. When molecules and sensors join together, microorganisms are surrounded and absorbed by the macrophage in a process known as phagocytosis. Phagocytosis stimulates macrophage to release cytokines which attracts neutrophils. Then, neutrophils absorb and destroy more microorganisms.

2. Specific. Immune exclusion: It basically consists in secretion of A immune globulin through the mechanism described above. Its function is limiting the primary focus and avoiding the transmission of it to prevent the introduction of the pathogen in the organism.