HLA
Meaning of HLA: The body's identity badge, read by T cells to tell its own healthy cells from infected, faulty or transplanted ones.
Definition of HLA
HLA is a set of proteins on the surface of the body's cells that display fragments of protein for the immune system to inspect. T cells read these fragments and judge whether the cell is healthy or holds something foreign. The pattern of HLA proteins varies enormously between people, which is why donor and recipient are typed before a transplant.
What do these proteins actually do?
Every cell chops up a sample of the proteins inside it and loads the pieces onto HLA molecules at its surface. Passing T cells inspect the display. A cell showing only its own familiar proteins is left alone. A cell showing a fragment from a virus, or an abnormal fragment from a tumor, is marked for destruction. This constant inspection is a core part of the immune response, and each displayed fragment counts as an antigen.
What are the two classes?
| Class I | Class II | |
|---|---|---|
| Named | HLA-A, HLA-B, HLA-C | HLA-DR, HLA-DQ, HLA-DP |
| Found on | Almost every cell with a nucleus | Cells that swallow material, such as macrophages and B cells |
| Shows | What is being made inside the cell | What has been taken in from outside |
| Read by | Killer T cells | Helper T cells |
Why is matching so hard?
The genes behind these proteins are the most variable in the human genome, with thousands of known versions at some positions. Each person inherits one set from each parent, so a full match is uncommon outside close relatives. A mismatch lets the recipient's T cells treat a graft as foreign, and in a bone marrow transplant it also lets donor cells attack the recipient. Tissue typing is a different problem from blood typing: the ABO blood group system offers a handful of options, while HLA offers an enormous number, so registries need very large numbers of volunteers.
What else is HLA connected to?
Particular HLA versions travel with a raised chance of specific conditions, among them celiac disease, type 1 diabetes and ankylosing spondylitis. Carrying such a version is common and is not a diagnosis on its own, since most carriers never develop the condition. What these links suggest is that the shape of the display influences which fragments the immune system reacts to, including fragments of the body's own tissue. A few HLA types are also tied to severe skin reactions to certain drugs, which is why testing is done in some populations before those drugs are started.
Also known as
Used in a sentence
HLA typing of the patient and two siblings was completed before donor selection for stem cell transplantation.
Good to know
Carrying an HLA type associated with a disease is not a diagnosis, because these types are common in healthy people, and HLA matching is a separate question from blood group matching, so both are checked before a transplant.