Most people know their ABO blood type. In some countries people carry a card indicating their blood type, in case of accident requiring an emergency blood transfusion. Few people however have heard of HLA types (human leukocyte antigens), the antigens in our blood that fight off microbes.
Contrarily to ABO blood types people do not have just one HLA type, but many (about 8 per person).
There are 3 major types of class I HLA (HLA-A, HLA-B, HLA-C) and 3 major types of class II HLA (HLA-DP, HLA-DQ and HLA-DR). Each type comprises hundreds of subtype (e.g. HLA-B27), further subdivided in hundreds of sub-subtypes (e.g. HLA-B*2705).
People will usually have 2 types of HLA-A, 2 of HLA-B, and 2 of HLA-C as well as 1 or 2 other types.
HLA types are encoded in the HLA gene on chromosome 6. HLA types are therefore hereditary, just like the ABO blood type.
HLA’s role in fighting diseases
Each type and subtype is more or less efficient in fighting off viruses and noxious bacteria. There are tens of thousands of possible combinations of HLA, which is why some people never get sick, while other constantly have a cold, or are prone to some types of infections, depending on what HLA combination they have.
HLA types found in tropical countries tend to differ a lot from those in temperate parts of the world, because the viruses found there are different. Some Africans have developed HLA that give them resistance to malaria, for instance. When the Europeans arrived in the Americas, bringing with them new viruses on the continent, the biggest part of the Native American population of North America was wiped out as they didn’t have the right antigens to fight off even the common cold.
But too aggressive HLA’s can also be bad for the body. Some HLA types are known to attack the body’s own cells, causing what is known as autoimmune diseases, in other words diseases caused by one’s immune system attacking one’s own body.
Autoimmune diseases linked to HLA types
Here are a few known or suspected associations between HLA types and autoimmune conditions :
Celiac disease (gluten allergy) : 95% of all celiacs have HLA-DQ2. 12% have HLA-DQ8.
Diabetes : The HLA types DR2, DR6 and DR11 are protective against Type 1 diabetes. The risk alleles are DR3, DR4 and DQ2.5. DR3 is linked to late-onset, whereas carriers of DR4 are at risk for early-onset Type 1 diabetes. People who carry both DR3 and DR4 types are at the highest risk and will develop diabetes the youngest.
HLA’s play a lesser role in Type 2 diabetes. There is a suspected link with HLA-Cw4, DR7, DR11 and DQA1, among others.
Graves’ disease : HLA-DR3 plays a significant role in the disease.
Hashimoto’s thyroiditis : strongly associated with HLA-DR5.
Lupus : weakly associated with HLA DR3, DR4, DR15 and DQA1.
Multiple Sclerosis : HLA-DRB1*1501 plays a role in the disease.
Myasthenia gravis : the main risk factors are HLA B8 and DR3 with DR1.
Narcolepsy : strongly associated with HLA-DQB1*0602. There is also an association with HLA DR2 and HLA DQ1.
Psoriasis : HLA-Cw*0602 is the main risk factor. HLA DR1 and DR7 may also play a role.
Rheumatoid Arthritis : HLA DR1, DR4, DR5, DR8 and DR12 are associated with the disease at various levels.
N.B. : HLA-DR11 is the short spelling for HLA-DRB1*11, just like HLA-C6 is short for HLA-Cw*06.