Final answer:
The major histocompatibility complex (MHC) is a collection of genes that code for MHC molecules found on the surface of all nucleated cells in the body. MHC molecules are responsible for presenting antigens to immune cells, triggering an immune response. The MHC is highly polymorphic, allowing for recognition of a large diversity of pathogens by the immune system.
Step-by-step explanation:
The major histocompatibility complex (MHC) is a collection of genes that code for MHC molecules found on the surface of all nucleated cells in the body. MHC molecules are responsible for presenting antigens to immune cells, triggering an immune response. The MHC is highly polymorphic, meaning that there is a high level of genetic variation in the MHC genes. This polymorphism allows for recognition of a large diversity of pathogens by the immune system.
In humans, the MHC genes are also known as human leukocyte antigen (HLA) genes. The MHC I genes (HLA-A, HLA-B, and HLA-C) determine compatibility for organ transplantation. The high level of polymorphism in these genes makes it unlikely for a randomly chosen donor to match a recipient's six-allele genotype, which is why a parent or sibling is often a better donor choice due to a higher likelihood of genetic match.
Monocytes, which are precursors to macrophages and dendritic cells, can be found in the blood and also express MHC molecules on their surface.