Factor XIII or fibrin stabilizing factor is a zymogen found in blood of humans and some other animals. It is activated by thrombin to factor XIIIa. Factor XIIIa is an enzyme of the blood coagulation system that crosslinks fibrin. Deficiency of XIII worsens clot stability and increases bleeding tendency.Human XIII is a heterotetramer. It consists of 2 enzymatic A peptides and 2 non-enzymatic B peptides. XIIIa is a dimer of activated A peptides. Function
Within blood, thrombins cleave fibrinogens to fibrins during coagulation and a fibrin-based blood clot forms. Factor XIII is a transglutaminase that circulates in human blood as a heterotetramer of two A and two B subunits. Factor XIII binds to the clot via their B units. In the presence of fibrins, thrombin efficiently cleaves the R37–G38 peptide bond of each A unit within a XIII tetramer. A units release their N-terminal activation peptides.Both of the non-covalently bound B units are now able to dissociate from the tetramer with the help of calcium ions (Ca2+) in the blood; these ions also activate the remaining dimer of two A units via a conformational change.Factor XIIIa (dimer of two active A units) crosslinks fibrins within the clot by forming isopeptide bonds between various glutamines and lysines of the fibrins. These bonds make the clot physically more durable  and protect it from premature enzymatic degradation (fibrinolysis).In humans, plasmin, antithrombin and TFPI are the most relevant proteolytic inhibitors of the active factor XIIIa. α2-macroglobulin is a significant non-proteolytic inhibitor.
A second study using CHOP-like regimens or one of these regimens plus methotrexate also produced better results with the latter regimens: 5 year survival rates for the CHOP-like and CHOP-like plus methotrexate regimens were 34.4% and 45.7%, respectively. A review of 105 PEL cases reported median survival times, 1 year, 3 year, and 5 year survival rates of 4.8 months, 30%, 18%, and 17%, respectively. In this study, patients with advanced Ann Arbor Stage III or IV disease had a particularly poor survival rate at 1 year of 25%; this compared to a rate of 42% for patients with stage I or II disease.Anti-viral drugs directed against Cytomegalovirus (i.e. cidofovir, ganciclovir, and valganciclovir) have been reported to produce complete presumed temporary responses in individual cases of PEL while drugs directed against HIV in patients with HIV+ PEL have achieved presumed temporary median response and 5 year survival rates of 0.7 months and 28%, respectively. The National Comprehensive Cancer Network (NCCN) guideline recommends treating HIV/AIDS-related PEL with antiviral therapy in combination with aggressive chemotherapy regimens such as DA-EPOCH, cyclophosphamide, doxorubicin, and etoposide, or CHOP. Rituximab, a monoclonal antibody directed against and killing CD20-expressing cells, appears to improve the efficacy of chemotherapy regimens in treating cases of PEL that evidence CD20-positive malignant cells such as Type II PEL. It has been suggested that regimens that include rituximab might improve the treatment of not only CD+ Type II PEL but also the uncommon cases of CD20+ Type I PEL and all cases of CD- PEL.