During HLA typing, the different classes of the major histocompatibility complex (MHC) are determined. The HLA system is the human version of the MHC that is present in many animals. The HLA classes are located on different gene loci on chromosome 6. These genes code for cell-surface proteins that regulate the immune system. The HLA genes are highly polymorphic: Many different alleles allow the fine-tuning of the adaptive immune system. The proteins encoded by those genes are also called antigens.
HLA class I is present on all human cells carrying a nucleus. However, the highest HLA class I concentration is found on lymphocytes and macrophages. HLA class I presents peptides from the inside of the cells. These peptides are digested and broken down in the proteasome and, subsequently, presented on the cell surface. These peptides usually have a length of eight to ten amino acids. However, not only own peptides are presented: if a cell is infected with a virus, the HLA system also presents digested virus peptides on the surface of the cell. This presentation of foreign peptides attracts immune cells, such as T-lymphocytes that destroy the identified, infected cells.
In contrast to HLA class I, HLA class II presents peptides from the outside to T-lymphocytes. Foreign pathogens are captured by so-called antigen-presenting cells and phagocytosis. Thus, HLA class II is present on, e.g., B-cells, activated T-cells, or macrophages. These cells digest the pathogen and present the peptides on the HLA class II. Activated T-lymphocytes trigger the specific antibody production of corresponding B-cells.