Quantitative White Blood Cell Disorders – White Blood Cell Pathology (WBC)

WBCs develop from stem cells in the bone marrow and are called leukocytes when circulating in the bloodstream. Lymphocytes are 1 of the 5 subclasses of WBCs. Lymphocytosis is an increase in the number or proportion of the lymphocyte subclass of WBCs, often as a result of an immune response to infection (known as reactive lymphocytosis). Common presentations include fever, lymphadenopathy, and upper respiratory symptoms. Usually transient, lymphocytosis resolves on its own in most cases after resolution of the infection. In other situations, it may signal a lymphoproliferative disorder (known as clonal lymphocytosis). Diagnosis is based on peripheral blood smear, flow cytometry, and potentially, a bone marrow biopsy. Management depends on the underlying cause.

Lymphadenopathy is lymph node enlargement (> 1 cm) and is benign and self-limited in most patients. Etiologies include malignancy, infection, and autoimmune disorders, as well as iatrogenic causes such as the use of certain medications. Generalized lymphadenopathy often indicates underlying systemic disease. Diagnosis relies on a thorough medical history and physical examination, which usually identify the underlying cause. When the cause is unknown, a biopsy may be necessary. Management is directed at the underlying infection, malignancy, or other cause.

Severe congenital neutropenia (SCN) affects myelopoiesis and has many different subtypes. SCN manifests in infancy with life-threatening bacterial infections. The treatment proven to be effective is the administration of granulocyte colony-stimulating factor, which elevates the decreased neutrophil count. Kostmann disease (SCN3) has an autosomal recessive inheritance pattern, whereas the most common subtype, SCN1, has autosomal dominant inheritance.

The myeloid lineage produces a diverse group of blood cells (leukocytes) involved in the defense of the body against foreign substances or infectious agents. Monocytes, neutrophils, eosinophils, basophils, and mast cells all arise from the common myeloid progenitor (CMP), which develops from bone marrow stem cells. However, leukocytes have different functions. Neutrophils kill invading bacteria and fungi, eosinophils increase in parasitic infections, basophils and mast cells are key to allergic response as sources of histamine, and monocytes are released from bone marrow to become macrophages in the tissues. The development in the bone marrow is regulated by different cytokines.

Neutrophils are an important component of the immune system and play a significant role in the eradication of infections. Low numbers of circulating neutrophils, referred to as neutropenia, predispose the body to recurrent infections or sepsis, though patients can also be asymptomatic. Most causes of neutropenia are acquired, including neutropenia that is secondary to infections, bone marrow disorders, and/or adverse effects of many drugs. The diagnosis is made using CBC and peripheral blood smears. Management is aimed at preventing and treating infections.