ABSTRACT
As it matures, the fetus develops its immune organs. Lymphocytes are generated in the primary lymphoid organs: the bone marrow, thymus, and intestinal Peyer’s patches.[2,3] T and B lymphocytes from these tissues then start circulating and eventually localize in peripheral or secondary immune tissues, where adaptive, or acquired, immune responses take place. Effective immune responses require immune cells to be localized in secondary lymphoid organs. The neonate requires time for its immune tissues to become mature. Because of their lack of immune system development, neonates are typically more susceptible than older animals to respiratory or intestinal infections. Probiotics have been developed to assist in maturing the intestinal immune tissues. Cytokines and chemokines serve as lymphoid tissue hormones and help to regulate immune system development and differentiation.[5]
Once a foreign antigen (i.e., an antigen produced from infectious microbes or vaccine preparations) enters the body, it is encountered by an APC-a dendritic cell or macrophage-and is transported to the local lymphoid organ, the lymph node, spleen, or specialized lymphoid tissues in the
Ho rm
on es
–
La m
b
gut or respiratory sites. In these secondary lymphoid sites the foreign antigen is presented by APCs to T and B cells and an acquired immune response is initiated. Many immune cells excrete a broad range of cytokines and chemokines, such as the interferons (IFNs) and interleukins (ILs), that activate the immune system and encourage cells to migrate and localize to the area of infection or tumor growth.[5]
MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) ANTIGENS
The major histocompatibility complex (MHC) antigens-or the swine, dog or bovine leukocyte antigens (SLA, DLA or BoLA)—are highly polymorphic, cell-surface antigens involved in antigen presentation.[4] [The international ImMunoGeneTics project (IMGT) maintains the HLA website and its IMGT/HLA Sequence Database. A related IPD/MHC sequence database website (https://www.ebi.ac.uk/ipd/mhc/ index.html) will be used for MHC sequences of veterinary species.] Class I MHC antigens are expressed on most cells, whereas class II MHC antigens are preferentially expressed on APC. The MHC genes are localized close together in the genome. Animals are usually MHC heterozygous, having two alleles at each of the multiple classes I and II genes.[3,4] Each animal expresses several class I MHC molecules, each of which is highly polymorphic. Class II genes are encoded by several linked loci, the DR and DQ alpha and beta genes. This wide diversity of MHC antigens is thought to be needed to handle the enormous number of foreign antigens that an animal encounters.
