Lactic acid bacteria and their effect on the immune system.

Lactic acid bacteria (LAB) are present in the intestine of most animals. The beneficial role played by these microorganisms in the humans and other animals, including the effect on the immune system, has been extensively reported. They are present in many foods and are frequently used as probiotics to improve some biological functions in the host. The activation of the systemic and secretory immune response by LAB requires many complex interactions among the different constituents of the intestinal ecosystem (microflora, epithelial cells and immune cells). Through different mechanisms they send signals to activate immune cells. Thus the knowledge of the normal intestinal microflora, the contribution of LAB and their role in the numerous functions in the digestive tract as well as the functioning of the mucosal immune system form the basis for the study and selection of a probiotic strain with immunostimulatory properties. In the selection of LAB by their immunostimulatory capacity it helps to know not only the effect which they have on the mucosal immune system, but the specific use to which these oral vaccine vectors are being put. Although there are reports of the protection of animals and humans against diseases such as microbial infections and cancer, more work remains to be done on the factors affecting the design of oral vaccine vectors and the use of LAB for therapeutic purposes. The basic knowledge of LAB immunostimulation and the criteria for selection of LAB by their immunostimulatory capacity, will be extensively discussed and appraised in this review.