A blood test showing reactive lymphocytes indicates an active immune response within the body. These specialized white blood cells, specifically lymphocytes, become activated and change in appearance when they encounter an infection or other inflammatory trigger. Their presence is a normal indication of a healthy immune system effectively responding to a threat, rather than signifying a disease itself.
Appearance and Identification of Reactive Lymphocytes
Lab technicians identify reactive lymphocytes by their distinct features when viewed under a microscope. Compared to smaller, resting lymphocytes, reactive forms are noticeably larger, often exceeding 16 micrometers in diameter. Their cytoplasm, the substance surrounding the nucleus, becomes more abundant and can appear deeply basophilic, staining a darker blue, or a pale blue-gray. This expanded cytoplasm may also show a characteristic scalloped edge where it presses against neighboring red blood cells.
The nucleus of a reactive lymphocyte is typically less condensed than in a normal lymphocyte and can have an irregular shape, appearing round, elliptic, indented, or folded. While nucleoli are usually subtle, they may occasionally be faintly visible. The overall variability in size, shape, and cytoplasmic features helps laboratory professionals recognize them as reactive.
Common Causes of Reactive Lymphocytes
The presence of reactive lymphocytes is most frequently linked to viral infections. Epstein-Barr virus (EBV), the cause of infectious mononucleosis, is a common culprit, leading to a marked increase in these activated cells. Cytomegalovirus (CMV) can also trigger a similar immune response, producing reactive lymphocytes that are morphologically indistinguishable from those seen in EBV infections. Other viruses, including influenza, hepatitis viruses, mumps, measles, rubella, and primary HIV infection, can also lead to their appearance.
Certain bacterial infections, such as Bordetella pertussis (whooping cough) or Bartonella henselae (cat-scratch disease), may also induce reactive lymphocyte changes. Beyond infections, other stimuli can activate lymphocytes, including drug reactions, certain autoimmune disorders like rheumatoid arthritis, or physiological stress.
Distinguishing From Malignant Cells
A common concern when reactive lymphocytes are detected is differentiating them from malignant cells, such as those found in leukemia or lymphoma. Reactive lymphocytes display polymorphism, meaning they exhibit a wide range of varied appearances in size, nuclear shape, and cytoplasmic characteristics, representing a diverse collection of immune cells responding to a threat.
In contrast, malignant lymphocytes typically appear monomorphic, meaning they are uniform and look very much alike because they are clones from a single cancerous cell. Laboratory professionals examine numerous cells on a blood smear to identify this spectrum of morphology, which helps confirm a reactive process. If morphological assessment is challenging, additional diagnostic tests like flow cytometry or a bone marrow biopsy may be performed to establish a definitive diagnosis.
Clinical Significance and Follow-Up
The finding of reactive lymphocytes serves as an important clue for a healthcare provider, indicating ongoing immune system activity. Their presence prompts the doctor to consider underlying causes, correlating blood test results with a patient’s symptoms and medical history. Symptoms like fever, sore throat, fatigue, or swollen lymph nodes might suggest an infectious cause.
In most instances, particularly when a clear viral illness is identified, the presence of reactive lymphocytes is temporary. As the body fights off the infection, these activated cells gradually return to their normal state, and their numbers decrease. If reactive lymphocytes persist without a clear explanation, or if a patient experiences other concerning symptoms, further evaluation or a referral to a hematologist may be recommended.