Tissue typing is a laboratory procedure that identifies specific protein markers on the surface of cells, primarily to determine compatibility between a potential organ donor and a recipient before transplantation. The goal is to minimize the risk that the recipient’s immune system will recognize the transplanted tissue as foreign and attack it. Identifying these markers allows clinicians to select the best possible match, which is a fundamental requirement for most solid organ and bone marrow transplants to prevent rejection.
Understanding Human Leukocyte Antigens
The biological basis for tissue typing centers on Human Leukocyte Antigens (HLA), the human version of the Major Histocompatibility Complex (MHC). These HLA molecules are present on the surface of most cells, functioning as identification tags for the immune system. They allow the immune system to distinguish between the body’s own cells (“self”) and foreign invaders (“non-self”).
The HLA genes, located on chromosome 6, are the most variable region in the human genome, a characteristic known as polymorphism. This high degree of diversity means thousands of variations, or alleles, exist across the population, making it rare for two unrelated individuals to share an identical set of HLA markers. An individual inherits half of their HLA genes from their mother and half from their father, which explains why full siblings have only a one-in-four chance of being a perfect match.
HLA molecules are divided into two primary groups, Class I and Class II, each having a distinct function and distribution. HLA Class I molecules (encoded by HLA-A, HLA-B, and HLA-C genes) are expressed on nearly all nucleated cells. Their function is to display small protein fragments made inside the cell to cytotoxic T-cells. If the fragment is from a virus or a foreign cell, the T-cell recognizes and destroys the cell.
HLA Class II molecules (encoded by genes like HLA-DR, HLA-DQ, and HLA-DP) are found primarily on specialized immune cells, such as B-cells and macrophages. These molecules present protein fragments derived from outside the cell (e.g., from bacteria) to helper T-cells. The helper T-cells then stimulate the immune response, including antibody production. Both HLA classes are highly immunogenic, meaning a mismatch can quickly trigger a severe immune attack against the transplanted organ.
Methods Used in Tissue Typing Analysis
Tissue typing has advanced significantly from older methods that relied on cell-based reactions to modern techniques analyzing a person’s DNA sequence directly. Historically, serological typing used known antibodies to detect antigens on the surface of lymphocytes. While quick, this method provided lower resolution and could not distinguish between subtle differences in HLA subtypes.
Today, the gold standard for high-resolution tissue typing involves molecular techniques, primarily using Polymerase Chain Reaction (PCR). PCR creates millions of copies of the specific HLA gene segments being tested, allowing for detailed genetic analysis. This amplification step is necessary because the amount of DNA in a single sample is too small for direct analysis.
The amplified DNA is then analyzed using advanced sequencing methods, such as Sequence-Based Typing (SBT). SBT directly reads the sequence of the nucleotide bases (A’s, T’s, C’s, and G’s) in the HLA genes, providing a precise, high-resolution identification of the specific HLA alleles a person possesses. Next-Generation Sequencing (NGS) is also becoming common, offering higher throughput and the ability to resolve complex or ambiguous results more efficiently.
These molecular techniques allow laboratories to identify HLA at the allele level, finding exact genetic matches rather than just broad antigen groups. High-resolution typing is particularly important for bone marrow transplants, where a single mismatch can lead to graft-versus-host disease. For solid organ transplants, high-resolution typing of the most important loci (A, B, and DR) is used to select the most compatible donor.
Assessing Match Quality and Compatibility
The results from the molecular typing analysis are used to calculate a compatibility score, reflecting the number of shared HLA antigens between the donor and recipient. For most solid organ transplants, three specific HLA loci—HLA-A, HLA-B, and HLA-DR—are considered the most relevant for predicting rejection risk. Since each person inherits two copies of each gene, a perfect match involves sharing all six of these antigens, often referred to as a 6/6 match.
While a perfect score is ideal, especially for kidney transplants, organs are often successfully transplanted with fewer matches due to immunosuppressive drugs. A mismatch at the HLA-DR locus is associated with a greater risk of rejection than a mismatch at the HLA-C locus. The match score is a prediction, not a final decision, as the recipient’s immune history must also be considered.
The Panel Reactive Antibody (PRA) test determines how sensitized a recipient’s immune system is to foreign HLA antigens. A high PRA percentage means the recipient has antibodies against a large portion of the general donor population, often due to prior transfusions, pregnancies, or previous transplants. For example, a PRA of 99% suggests the recipient would likely react positively against 99% of randomly selected donors, making finding a compatible organ challenging.
The final compatibility test performed immediately before a transplant is the crossmatch. This test involves mixing the recipient’s serum (containing antibodies) with the donor’s lymphocytes (carrying HLA antigens). A “positive” crossmatch means the recipient’s antibodies attacked and killed the donor’s cells, indicating a high risk of immediate organ rejection. A positive result usually contraindicates proceeding with the transplant, while a “negative” crossmatch suggests the transplant can proceed with a lower risk of hyperacute rejection.