Hemophilia is a blood disorder that’s usually genetically inherited from parents. People born with hemophilia have little or no clotting factor – a protein needed for normal blood clotting.
Hemophilia can also be acquired, meaning patients don’t inherit the disease from their parents but instead develop it during their lifetime. This can happen if their body forms antibodies that attack the clotting factors in the blood.
Both hemophilia A and B are passed down from a parent to a child. The genes for the disease are on the X chromosome, meaning that when the father has hemophilia but the mother doesn’t and is not a carrier, sons will have hemophilia. Daughters, however, will carry the hemophilia gene.
If a woman inherits a copy of the faulty gene from one of her parents, she is said to be a “carrier” of hemophilia. She has one normal gene and one faulty gene, and she can pass one of the two onto her children. There is a 50 percent chance that her sons will develop the disease and a 50 percent chance or that her daughters will be a carrier of the disease. For daughters to have the disease, their father must also have hemophilia.
On average, carriers of hemophilia have about half of the normal amount of clotting factor, but some carriers have far lower levels than that.
Epidemiology of hemophilia
Hemophilia A is seen in about 1 in every 4,000–5,000 males worldwide, compared with hemophilia B, which is estimated to be in 1 in every 20,000 men. Hemophilia C, a much rarer form of the disease, is estimated to occur in about 1 case per 100,000 people in the U.S.
Each year, about 400 babies are born with some type of hemophilia in the U.S., and it is thought there are around 20,000 people living with the disease in the country, most of whom have a severe form of the disease.
Hemophilia affects all ethnic groups equally throughout the world. Although men are mostly affected by both hemophilia A and B, about 30 percent of female carriers have clotting factor levels low enough to be in the hemophilia range. This is due to a phenomenon called lyonization or X chromosome inactivation, where one of the two X chromosomes in each cell is randomly inactivated.
If the X chromosome that is mostly inactivated carries the normal hemophilia gene, carrier women may experience bleeding or other symptoms associated with the disease. Severe hemophilia rarely occurs because of extreme lyonization, homozygosity (inheriting the faulty gene from both parents), mosaicism (having cells of two genetically different types), or Turner syndrome, where one of the X chromosomes is partially or totally missing.
Hemophilia C affects women and men equally.
Acquired hemophilia is estimated to affect around one to three people per million. Both genders and all ethnic groups are equally affected in this case.
Etiology of hemophilia
There are several types of clotting factors and each type of hemophilia refers to one type of clotting factor deficiency.
Hemophilia A is a deficiency of clotting factor VIII, also known as anti-hemoplytic factor/antihemophilic factor (AHF) or globulin (AHG)/antihemophilic factor A, produced in the endothelium lining blood vessels.
Hemophilia B is a deficiency of clotting factor IX, also called Christmas factor / plasma thromboplastin component (PTC) / antihemophilic factor B, produced in the liver.
Hemophilia C is a rare deficiency of clotting factor XI, also known as plasma thromboplastin antecedent (PTA) / antihemophilic factor C, produced in the liver.
Clotting factors are proteins in the blood that control bleeding. Coagulation factors circulate in the blood in an inactive form. When a blood vessel is injured, the coagulation cascade is initiated and each coagulation factor is activated in a specific order to lead to the formation of a blood clot.
Acquired hemophilia is, in the vast majority of cases, related to age or to childbirth and the condition often resolves with appropriate treatment.
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