White blood cells (WBCs), or leukocytes, are the body’s immune system defenders, constantly circulating to protect against infection and disease. A standard complete blood count (CBC) includes a differential count, which measures the five common types of mature WBCs: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. When a blood sample is analyzed, the term “rare WBCs” or “atypical cells” often appears on the manual differential report. This notation signals that cells were observed that do not fit the criteria of the five typical, mature leukocytes. The presence of these unusual cells prompts further investigation to understand what they represent and why they are circulating.
Understanding How “Rare” is Determined
The detection of rare white blood cells results from a two-step laboratory process involving automated counting and manual review. Initial analysis is performed by an automated hematology analyzer, which quickly counts thousands of cells and provides percentages of the five main WBC types. If the machine detects cells that fall outside the normal size, shape, or complexity parameters, it flags the sample for manual review.
The official finding of “rare WBCs” comes from a manual differential, where a trained technician or pathologist examines a stained blood smear under a microscope. In this context, “rare” is a quantitative term, meaning the cells are present in numbers too low to be automatically counted or included in the standard differential count. For instance, “rare” might mean fewer than one or two unusual cells are observed across multiple high-power fields of the slide. This finding is significant because even a few unusual cells can indicate an underlying biological process requiring clinical attention.
Identifying the Unusual White Blood Cells
The cells most frequently flagged as “rare” are either reactive (responding to a stimulus) or immature (released from the bone marrow too early). One common type is the Atypical Lymphocyte, described as a large, reactive cell with abundant cytoplasm that often appears “scalloped.” These cells are usually activated cytotoxic T cells that multiply and change appearance while fighting a viral infection. Their varied morphology, encompassing a range of sizes and shapes, is a hallmark of a reactive, non-cancerous process.
Immature Granulocytes
Another category of rare cells is Immature Granulocytes, which include stages like myelocytes and metamyelocytes. Normally, these cells mature in the bone marrow before entering the peripheral blood, so their presence indicates a “left shift” in production. Immature granulocytes are larger than mature neutrophils and possess a round or kidney-bean-shaped nucleus, unlike the lobed nucleus of mature cells.
Blasts
A more concerning finding is the presence of Blasts, which are highly immature progenitor cells and the earliest recognizable form of blood cells. Blasts are characterized by a high nucleus-to-cytoplasm ratio, fine chromatin, and sometimes prominent nucleoli. Their uncontrolled appearance in the blood is a sign of a severe hematologic disorder.
Common Reasons These Cells Appear
The presence of rare white blood cells falls into two main groups: those resulting from a reactive, temporary state and those stemming from a serious hematologic condition. Reactive causes are the most common explanation for atypical lymphocytes or immature granulocytes. Viral infections, such as Epstein-Barr virus (Infectious Mononucleosis) or Cytomegalovirus (CMV), stimulate the immune system to produce atypical lymphocytes. Severe bacterial infections or significant stress can also trigger the bone marrow to rapidly release immature granulocytes in response to inflammation. These reactive changes are typically transient and resolve once the underlying cause has passed.
In contrast, the appearance of blasts, or a persistent high number of immature cells, often points toward a primary bone marrow disorder. Conditions like Myelodysplastic Syndrome (MDS) or various forms of Leukemia are characterized by the abnormal production and release of these highly immature cells. Leukemia involves the uncontrolled proliferation of blasts that crowd out the normal production of mature blood cells. When immature cells are found, the clinical picture is assessed, including whether the patient is also experiencing anemia or a low platelet count, which suggests bone marrow failure.
Interpreting the Finding and Follow-Up
The clinical significance of rare white blood cells is determined by interpreting the finding within the context of the patient’s overall health and the complete blood count results. If the total WBC count is elevated and the rare cells are atypical lymphocytes, the finding is likely a benign, reactive response to a recent infection. A single finding of a few rare cells in an otherwise healthy individual often leads to repeating the blood test in a few weeks to ensure the cells have cleared.
If the rare cells are identified as blasts, or if the patient has concerning symptoms like unexplained fatigue, fever, or bruising, the finding requires immediate, specialized follow-up. The next steps may involve ordering a flow cytometry test, which uses specialized markers to precisely identify the cell lineage and maturity of the unusual cells. When a serious hematologic condition is suspected, a referral to a hematologist is made, and a bone marrow biopsy may be performed to directly examine the blood cell production factory.