Colony stimulating factors (CSFs) are a group of proteins your body naturally produces to signal the bone marrow to make new blood cells. They control which types of blood cells get made, how many, and how well those cells function once they’re mature. In medicine, lab-made versions of these proteins are widely used to boost white blood cell counts in people undergoing chemotherapy, bone marrow transplants, or recovery from radiation exposure.
How CSFs Work in the Body
Deep inside your bone marrow, stem cells sit in a quiet, resting state until they receive chemical signals telling them to wake up and start dividing. Colony stimulating factors are among the most important of those signals. They don’t just flip a switch to produce more cells. They guide the entire process: which type of blood cell a stem cell becomes, how quickly it multiplies, whether it survives or dies, and how effectively it works once it enters the bloodstream.
Once mature blood cells are released, CSFs continue to influence their behavior. They affect how immune cells move toward infections, how effectively they engulf bacteria, and how strongly they respond to threats. In healthy people, CSFs keep blood cell production in a steady balance. When infection or injury strikes, the body ramps up CSF production to meet the demand.
The Main Types of CSFs
There are four major colony stimulating factors, each with a somewhat different job. Two of them are relatively specific, targeting narrow cell lines. The other two cast a wider net, acting on earlier-stage stem cells that can develop into many different blood cell types.
- G-CSF (granulocyte colony stimulating factor) primarily drives the production of neutrophils, the white blood cells that serve as the immune system’s first responders against bacterial infections. It also enhances the bacteria-killing ability of mature neutrophils already circulating in the blood. G-CSF is produced naturally by immune cells, connective tissue cells, and the cells lining blood vessels.
- M-CSF (macrophage colony stimulating factor) focuses on producing macrophages, the large immune cells that swallow and digest pathogens and dead cells. Like G-CSF, it’s relatively lineage-specific, meaning it works on a narrow range of cell types and their precursors.
- GM-CSF (granulocyte-macrophage colony stimulating factor) acts earlier in the development process, before stem cells have fully committed to becoming one cell type. It boosts production of neutrophils, macrophages, and eosinophils (white blood cells involved in allergic responses and parasite defense). GM-CSF is found in T cells, immune cells, connective tissue, and blood vessel walls.
- IL-3 (interleukin-3), also called multi-CSF, has the broadest reach of any colony stimulating factor. It can stimulate the production of nearly every type of blood cell: macrophages, neutrophils, eosinophils, basophils, mast cells, platelets, and red blood cells.
Why CSFs Are Used in Medicine
Chemotherapy kills fast-dividing cells, which includes cancer cells but also the stem cells in bone marrow that produce white blood cells. This often causes a condition called neutropenia, a dangerous drop in neutrophil counts that leaves patients vulnerable to life-threatening infections. When neutropenia is accompanied by fever, the situation becomes urgent.
Lab-made versions of G-CSF and GM-CSF are given to patients to speed up neutrophil recovery. Clinical trials have shown that both G-CSF and GM-CSF reduce the incidence of febrile neutropenia when given shortly after chemotherapy. In patients who already have the condition, these drugs shorten hospital stays and speed the time it takes for neutrophil counts to bounce back. That said, the American Society of Clinical Oncology does not recommend routine CSF use for every case of febrile neutropenia. The decision depends on the patient’s risk level and the intensity of the chemotherapy regimen.
CSFs also play a critical role in stem cell transplants. Before a transplant, donors (or patients themselves) receive G-CSF injections that coax stem cells out of the bone marrow and into the bloodstream, where they can be collected through a blood draw rather than a painful bone marrow harvest. This process, called stem cell mobilization, sometimes combines G-CSF with chemotherapy drugs or other mobilizing agents to improve the yield.
A less common but important use: CSFs are approved for treating people exposed to high levels of radiation. In that scenario, the bone marrow is severely damaged, and CSF injections help it recover faster.
Available CSF Medications
The G-CSF drugs available in the United States include filgrastim (brand name Neupogen), pegfilgrastim (Neulasta), and several biosimilar versions. All are made using recombinant DNA technology, meaning the human gene for G-CSF is inserted into bacteria, which then produce the protein in large quantities. Pegfilgrastim is a modified form of filgrastim designed to stay in the body longer, so it only needs to be given once per chemotherapy cycle instead of daily.
For GM-CSF, the only available drug is sargramostim (Leukine), which is produced using yeast cells rather than bacteria. Because GM-CSF acts on a broader range of white blood cells than G-CSF, sargramostim boosts not just neutrophils but also macrophages and eosinophils.
How CSF Injections Are Given
CSFs are given as injections under the skin. For the long-acting form, pegfilgrastim, a single 6 mg injection is given once per chemotherapy cycle. Timing matters: it should not be given within 14 days before or 24 hours after chemotherapy, because stimulating bone marrow division at the same time as chemotherapy could damage the very cells you’re trying to protect.
The short-acting form, filgrastim, requires daily injections, typically starting a day or two after chemotherapy and continuing until neutrophil counts recover. For children weighing under 45 kg, doses are adjusted by weight rather than given as a flat dose.
In radiation emergencies, the protocol is different: two injections of pegfilgrastim given one week apart, starting as soon as possible after exposure to radiation levels above 2 gray.
Side Effects
Bone pain is the most common side effect of G-CSF treatment, and it makes intuitive sense. The drug is pushing your bone marrow into overdrive, rapidly producing new cells inside the rigid confines of your bones. In clinical trials comparing pegfilgrastim and filgrastim in cancer patients, about 46 to 52% of patients reported bone pain during the first chemotherapy cycle. The rates were similar between the two drugs.
The good news is that the pain tends to lessen over time. By the fourth cycle, only about 24% of patients reported it. Severe bone pain (grade 3 or 4 on the clinical scale) was uncommon, occurring in roughly 4 to 5% of patients during the first cycle and dropping below 2% in later cycles. For most people, over-the-counter pain relievers are enough to manage it.
Injection site reactions, mild fevers, and muscle aches can also occur but are generally manageable. The side effect profile of GM-CSF is similar, though it can occasionally cause fluid retention or mild allergic reactions because of its broader immune-stimulating effects.