White Blood Cells (WBCs) are our defenses, to fight disease. They’re made in our bone marrow, which is the core part of certain bones (mainly the pelvis, sternum, and long bones). The common laboratory blood test Complete Blood Count (CBC) counts each kind of WBC, summarized in general as:
- Neutrophils (the main type of polymorphonucleocyte, called PMNs or Polys for short) — these attack bacteria. A high count means the body needs to make lots, so maybe there’s a bad bacterial infection. Counts >20,000 suggest Sepsis. A very low count <500 (“neutropenia”) means the body’s at risk of bacterial infection. Counts >50,000 make us worry about leukemia (blood cancer).
- Lymphocytes (or “Lymphs” for short) — these control viruses, but counts are often normal or even low with viral infections. Large “atypical” lymphocytes make us think of Mononucleosis. A very low lymphocyte count makes us wonder about HIV (which destroys a kind of Lymph). An extremely high count (>30,000) makes us think of certain cancers, but Pertussis (whooping cough) can also cause it. In order to count the CD4 cells for HIV, a CBC isn’t enough; we need to order a special test for it (costs more).
- Eosinophils — an uncommon WBC; if it’s significantly elevated, we think of allergies, asthma, certain tropical parasites, & maybe certain uncommon blood cancers
- “Bands” — an immature Neutrophil. If there are a lot of these, the body is trying to make neutrophils, so we suspect a bacterial infection may be going on.
- Other — some WBCs are only found in the bone marrow: metamyelocytes and myelocytes. That’s because they evolve into bands and mature WBCs before being released to the blood. But if we see them in a blood specimen we worry about Leukemia, although occasionally we see one or two during severe infection or extreme physical stress. A WBC called a “blast” means leukemia.
Often, we get all the information we need to know about WBCs from the simple CBC. Sometimes we need to order more sophisticated blood tests. like to count the different types of Lymphocytes for HIV patients. In order to diagnose Leukemia, and certain blood infections, we have to extract a sample of the bone marrow itself.
It’s important to note that we can also see WBCs in other specimens, like urinalysis, spinal fluid, joint fluid, etc. The more WBCs present, the more we realize there’s some sort of infection, because the immune system is sending WBCs there for some reason. Indeed, if we see bacteria on a urine culture, without WBCs in the urinalysis, we assume the bacteria are not causing an infection (they may just be contaminants from the skin or vagina). In spinal fluid, if the WBCs are neutrophils, it suggests bacterial meningitis (immediately life-threatening) whereas if they’re mainly lymphocytes, we suspect a viral cause (may be serious, but less immediate).