What is it?
Haemophilia is a genetic disorder that makes the body unable to stop bleeding because it can’t form blood clots. When healthy people are injured they stop bleeding after a short while, as these fibrous clots are produced and they block blood’s way out of the body. In haemophilia a certain factor that is necessary for this clotting is missing, and the affected can bleed for weeks from a small wound. This is particularly dangerous if it happens internally in vital body parts, such as the brain.
There are two main types of haemophilia: A and B. Haemophilia A is occurs in about 1 in 5000-10 000 male births, while haemophilia B occurs in around 1 in 20 000-34 000 male births.
What causes it?
Haemophilia is caused by one gene. We get a set of chromosomes from each of our parents, so we have two corresponding versions of nearly all our genes. These versions can work together, or one can be louder than the other. Females have two X chromosomes, while males have one X and one Y. The Y chromosome is basically an X chromosome with an “arm chopped off”. No matter how silent the version of a gene is that ends up in the corresponding part of a male’s X chromosome, it manifests itself, because there is no other gene to cover it in the Y chromosome.
This is why haemophilia almost exclusively occurs in men. The gene that causes it is recessive and is located in the X chromosome, which means that females need two copies but males only one to become affected.
What happens during it?
When our tissues are damaged and start to bleed, a complex series of reactions begins. First our vessels contract to minimize bleeding. We have lots of cell-like entities called platelets in our blood, and soon they start to excrete sticky substances which makes them attach to one another. We also have lots of a soluble protein called fibrinogen in our blood, and platelets produce substances that make these molecules turn into insoluble fibrin. Together fibrin and the sticky platelets form a clot, which effectively prevents bleeding until the wound is healed.
The reaction cascade that starts with tissue damage and leads to clot formation is long and complex, but essentially haemophilia inhibits clot formation by not producing a protein whose job it is to activate an enzyme to turn fibrinogen into fibrin. The body’s attempts to build clots is insufficient, and there’s nothing to stop blood from exiting the body until a scab has formed. It is relatively common for haemophilia patients to bleed into their joints, which can cause pain and deformity.
How can it be treated?
Symptoms can be prevented by injecting the missing protein regularly into the blood stream. This way clotting can happen normally. Injections can also only be given when an injury has happened. It is obviously wise for people with haemophilia to avoid situations where they might end up bleeding, such as contact sports.
Sources: https://en.wikipedia.org/wiki/Haemophilia, http://www.nhs.uk/Conditions/Haemophilia/Pages/Treatment.aspx, http://emedicine.medscape.com/article/779322-overview?pa=eXF6%2F4WNImmy%2BAObEPKf4IcNK%2B0l5LRvaiFRyL45cPr4fmJsRqD42wsCufHtLeHk56MI7dGTgNawPfsOtJla9Q%3D%3D#a3, https://en.wikipedia.org/wiki/Coagulation#Role_in_disease