Final answer:
The Major Histocompatibility Complex (MHC) is a collection of polymorphic genes expressing glycoproteins on nucleated cells, which help the immune system recognize self from non-self. Due to linkage and polygeny, these genes are often inherited together, significantly affecting organ transplant compatibility. A monocyte, while not part of the MHC itself, becomes a cell that interacts with MHC in the immune response.
Step-by-step explanation:
The Major Histocompatibility Complex (MHC) is a vital component of the immune system, consisting of a gene cluster whose proteins present antigens to T cells. MHC molecules are expressed on the surface of all nucleated cells of the body, allowing the immune system to recognize body cells as self. This gene cluster is highly polymorphic, meaning that there are multiple alleles for each individual MHC locus, referred to as MHC polygeny. Each person inherits a set of these genes, which include three highly polymorphic MHC I genes in humans (HLA-A, HLA-B, and HLA-C).
Due to the close proximity of the MHC genes on the chromosome, the phenomenon of linkage means these genes are often inherited together. With so many alleles present in the population, it becomes very unlikely that a randomly chosen donor will match another individual's specific six-allele genotype, which is crucial for organ transplantation to minimize the risk of rejection. As a result, family members often make better donors due to higher chances of a genetic match.
A monocyte is a precursor to macrophages and dendritic cells seen in the blood, and while these cells participate in the immune response, they aren't directly part of the MHC, but they do interact with MHC molecules during antigen presentation to T cells.