Human Leukocyte Antigens (HLA) are proteins on the surface of most body cells. They help the immune system distinguish between its own cells and foreign invaders like viruses or bacteria. Understanding an individual’s HLA profile is important in various medical contexts. This article explains the process of HLA testing.
Understanding Human Leukocyte Antigens
Human Leukocyte Antigens are a group of proteins that act like molecular identification tags on cells. The immune system uses these HLA markers to recognize the body’s own cells and identify foreign invaders, triggering an immune response against perceived threats.
These proteins are encoded by a complex of genes located on chromosome 6, known as the major histocompatibility complex (MHC). This genetic system is highly diverse, meaning there are many different possible variants of HLA genes among individuals. This diversity allows the immune system to respond to a broad range of pathogens. Individuals inherit their HLA type from their parents, receiving half from each.
Why HLA Testing is Performed
HLA testing is primarily performed to ensure compatibility between a donor and a recipient in organ and tissue transplantation. A close HLA match significantly reduces the risk of the recipient’s immune system rejecting the transplanted organ or tissue. This matching process is important for the success of procedures like bone marrow, cord blood, or solid organ transplants.
Beyond transplantation, HLA testing can investigate associations with certain diseases. Specific HLA types have been linked to an increased risk for autoimmune conditions, such as celiac disease, ankylosing spondylitis, and type 1 diabetes. HLA testing may also be relevant in pharmacogenetics, predicting how an individual might react to certain medications due to associations with adverse drug reactions. HLA typing can also be used in paternity testing to establish biological relationships, though this is a less common application.
The HLA Testing Process
The process of HLA testing begins with collecting a biological sample. The most common sample type is a blood sample, but buccal (cheek) swabs can also be used. The collected sample is sent to a specialized laboratory for analysis.
Inside the laboratory, DNA is extracted from the collected cells. This DNA is used to identify specific HLA types. After DNA extraction, Polymerase Chain Reaction (PCR) is used to amplify the specific HLA genes of interest. This ensures enough genetic material for accurate analysis.
Following amplification, various molecular typing methods identify the precise HLA alleles. Common methods include Sequence-Based Typing (SBT), which involves directly sequencing the DNA of the HLA genes to determine their exact nucleotide sequence. SSOP methods use small DNA probes that bind to specific HLA gene sequences, with the binding pattern indicating the HLA type. Next-Generation Sequencing (NGS) is also increasingly used, offering high-throughput and high-resolution HLA typing by sequencing large amounts of DNA.
The level of detail from HLA testing is called “resolution,” which can be low, intermediate, or high. Low-resolution typing identifies broader groups of HLA alleles, while high-resolution typing provides a more detailed identification of individual alleles. The choice of resolution depends on the specific clinical need, with high resolution often essential for the best possible match in transplantation. Specialized software then analyzes the genetic data to assign specific HLA types by comparing the sequenced DNA or probe binding patterns to a database of known HLA alleles.
Understanding Your HLA Test Results
HLA test results are reported using a standardized nomenclature that identifies specific HLA types or alleles. These results appear as a combination of letters and numbers, such as “HLA-A\01:01” or “HLA-B\08:01”. The numbers after the asterisk denote specific allele groups and then more precise variations within those groups.
These detailed results are used in different ways depending on the reason for testing. In transplantation, a closer match in HLA types between a donor and a recipient leads to a lower risk of transplant rejection and a greater chance of success. For disease association, the presence of certain HLA types can indicate an increased genetic predisposition to particular conditions. However, such associations indicate a higher risk, not a certainty of developing a disease.
Interpreting HLA test results can be complex due to the vast diversity of HLA types and their intricate roles in the immune system. Therefore, these results should always be reviewed and explained by a qualified medical professional or genetic counselor. They can provide context, discuss the implications for an individual’s health, and guide any further medical steps.