The Biology of Birdshot & Your Genes

Understanding the biology of Birdshot Chorioretinopathy (BSCR) can help you make sense of why your body is reacting this way. It is not caused by an infection, a toxin, or anything you did; rather, it is a complex condition involving your genetics and your immune system ^[1].

The Role of HLA-A29

The most significant biological factor in Birdshot is a genetic marker called HLA-A29.

• What is it? HLA-A29 is an immune system protein found on the surface of your cells. Its normal job is to show “snippets” of proteins to your immune system so it can check for viruses or bacteria ^[2].
• The Association: The link between Birdshot and HLA-A29 is one of the strongest in all of medicine. One major study found that people with Birdshot are over 50 times more likely to carry this gene than those without the disease ^[3]. In fact, nearly every patient diagnosed with Birdshot is HLA-A29 positive ^[4][5].
• Susceptibility, Not Certainty: Having the HLA-A29 gene does not mean a person will definitely get Birdshot. It is a relatively common gene, but only a tiny fraction of carriers ever develop the disease ^[1]. Because of this, it is called a “susceptibility marker.” It provides the possibility of the disease, but does not cause it on its own.

Should my family be tested? Because the gene is common and the disease is extremely rare (low penetrance), routine genetic testing of asymptomatic family members (children, siblings) is generally not recommended and can cause unnecessary anxiety. Relatives should simply seek an eye evaluation if they ever develop persistent visual symptoms.

Research Hypotheses: How the Immune Attack Happens

Scientists do not yet know the exact trigger that causes Birdshot, but there are several leading research hypotheses about how the disease mechanism works:

• Peptide Trimming (ERAP1 and ERAP2): Genes called ERAP1 and ERAP2 produce enzymes that trim proteins to fit into the HLA-A29 “display case.” Researchers hypothesize that in Birdshot patients, these enzymes may mistakenly cut “self” proteins from the eye into shapes that the immune system perceives as dangerous ^[2][6]. (Note: ERAP testing is purely for research and does not currently guide clinical treatment).
• T-Cells and Th17: The immune attack involves specific white blood cells, such as CD8+ T-cells, and inflammatory messenger chemicals known as Th17 cytokines (like IL-17 and IL-23). These cells and chemicals are found in higher amounts in the eyes of Birdshot patients, leading researchers to believe they drive the chronic inflammation ^[7][8].

Why You?

Researchers are still trying to figure out the final “trigger” that causes the disease to start in a person who has carried the HLA-A29 gene their whole life. Various theories include hormonal changes, unrecognized viral exposures, or changes in the gut microbiome ^[9][10]. However, these remain unproven research associations, not established facts that can predict an individual’s disease course.

The Bottom Line: A positive HLA-A29 blood test is a vital clue for your doctor, but it is not a diagnosis by itself ^[11]. Your diagnosis is based on what the doctor sees in your eyes—the genetic marker simply helps explain the susceptibility behind the inflammation ^[1].