Final answer:
The combination of all HLA class I and II allotypes expressed by an individual is known as their major histocompatibility complex (MHC). It includes glycoproteins that play a vital role in the immune response and antigen presentation.
Step-by-step explanation:
The combination of all HLA class I and class II allotypes that an individual expresses is referred to as their major histocompatibility complex (MHC).
The major histocompatibility complex refers to a collection of genes that code for MHC glycoproteins. These proteins are expressed on the surface of all nucleated cells. The MHC plays a crucial role in the immune system by presenting peptides derived from pathogens or abnormal cells to T cells. It signifies whether a cell is healthy and normal or is displaying signs of infection or malignancy. The MHC molecules can be categorized into two classes. Class I MHC molecules are found on the surface of all nucleated cells and are essential for presenting antigens to CD8+ T cells, while class II MHC molecules are primarily expressed on professional antigen-presenting cells (like dendritic cells, macrophages, and B cells) and are necessary for presenting antigens to CD4+ T helper cells.
People have three highly polymorphic MHC Class I genes, HLA-A, HLA-B, and HLA-C, with numerous alleles segregating in a population. The six alleles (two from each gene due to codominant expression) make up a unique MHC genotype that is extremely unlikely to match perfectly with a randomly chosen donor, hence the importance of family members when looking for transplant donors to reduce the risk of organ rejection.