What exactly is blood?
Blood is made up of many different components that play vital roles in the body like maintaining homeostasis, fighting out infections, and carrying oxygen and carbon dioxide to various tissues and organs in the body. Blood is very important to your body and to the leukapheresis process.
In simplest terms, leukapheresis produces leukocytes, or white blood cells. Certain apheresis technology, however, can target specific parts of the total white blood cell count known as mononuclear cells.
This diagram delineates how blood is broken down and where leukapheresis plays its role.
The highlighted arrows directs the path of WBC lineage that goes all the way to the mononuclear level (i.e. lymphocytes and monocytes) and multi-lobed nuclei level (neutrophils, eosinophils, and basophils)
Origin of Blood Cells
This diagram gives a different look on the lineage of WBCs . Note that WBCs derive from granulocytemonocyte precursors, a relationship that is again seen in the breakdown of phagocytes from the Blood Breakdown diagram. Also note that lymphocytes, a type of WBC, comes from a different precursor (lymphoid progenitor cells) than the phagocytes (myeloid progenitor cells), thus prompting different roles within the immune system.
Exactly how much blood is processed and how many WBCs is obtained?
Early definitions of how much blood circulates the human body state that the average person carries about 5 L of blood when they reach adulthood . Now, most records report that the average person, weighing in around 160 lbs, has about 5 quarts, or 4.7 L, of blood . Researchers will refer to this 4.7 L or 5 L value as one “blood volume”, meaning that this is the volume of blood for one individual.
This “blood volume” reference is important to leukapheresis because the most up-to-date reviews on apheresis techniques recommend a starting point of two blood volumes to be processed, although each machine will slightly differ in the customizable approach to the technique. This means that the leukapheresis procedure will continue until the individual’s entire volume of blood has been processed not once, but twice. Researchers have found that more of the component (or desired target-WBC) can be removed with maximum efficiency with two blood volumes. Basically the Goldilocks principle comes into play: one blood volume does not produce enough component (63%), two blood volumes produces a much higher yield (87%), and three blood volumes produces similarly to two blood volumes, making it inefficient. Although studies have shown that if patients can tolerate the procedure well, additional volumes can be processed (i.e. for patients undergoing therapeutic leukapheresis as opposed to participants donating WBC for research).
On average, 600 ml of WBC product is obtained from a single leukapheresis procedure. The efficiency of WBC separation will depend on the individual: the WBCs tendency for separation, the natural WBC preapheresis, and even the gravitational pull of the centrifugation process can all play a part in how much of the component will be removed.
The Blood Breakdown diagram shows the lifespan and regeneration highlights of some of the blood components. Note that the lifespan and regeneration timeline of a RBC versus a WBC is vastly different. Erythrocytes have a lifespan of 120 days with a very specific plan of action for “retiring” the worn out cells while granulocytes, a type of phagocytic leukocyte, have a lifespan of 21 days and require constant replenishing due to high turnover. This, along with the processed volume component, is why the RBC components must be returned back to the body and the WBC components can easily and safely be removed.
A single leukapheresis procedure can remove 20-50% of the peripheral WBC count and can be repeated multiple times depending on the physician order and course of treatment. Leukapheresis products, once collected, are then processed ex vivo through centrifugation and filtration for further purification. Once processed, the product can be stored on a short-term basis.
Storage of Product
According to AABB Temperature Standards, apheresed WBCs can be stored at 20°-24°C for a 24 hour period. This temperature range would be for immediate use either in research for test kits or transfusion (depending on the purpose of the procedure). However, WBC products can be frozen, within 24 hours of collection, at temperatures <-65°C in a monitored storage facility on a short-term basis (i.e. 12 months or less, if processed). This is unlike storage of frozen RBCs, which can be frozen at temperatures <-65°C for 10 years or more, according to the AABB.
The outlined storage of the leukapheresis product is vital to the maintenance of antifungal activity. Some findings have shown an increase in fungal infections in patients receiving transfused leukapheresis products. Even though these findings were limited to immunocompromised patients, proper storage will help prevent unwanted and potentially harmful fungal infections.
Improving Product Yield
Each individual’s baseline WBC count and tendency for separation will differ. Some facilities utilize different agents to help boost the number of peripheral WBC and improve the yield per treatment. Steroidal stimulants like dexametasone and prednisone increase granulocyte counts and thus improves the overall WBC yield while newer stimulants like G-CSF (granulocyte-colony-stimulating factor) are used as an injectable addition to steroids in order to increase the yield three to sometimes five-fold, though the long-term safety and economics have not been studied.
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