Classical hemophilia, often referred to as hemophilia A, results from a lack of antihemophilic globulin, or factor A. This is a sex-linked hereditary disease carried by females but affecting only male children. Hemophilia B results from a lack of plasma thromboplastin component, or antihemophilic factor B, and is also called Christmas disease. It is usually considered to have less serious potential than hemophilia A.
Source: wiktionary