Chapter 441

White blood cells, or leukocytes, are part of the innate immune system. They are one of the body’s major defenses in preventing and combating infection. The most common types of leukocytes are lymphocytes, monocytes, and granulocytes. This chapter will deal primarily with granulocytes. These cells are derived from stem cells in the bone marrow and have diverse functions. The most common of the leucocytes is the neutrophil. Deficiency of neutrophils, called neutropenia, is one of the most common hematologic abnormalities during childhood and when severe can result in life-threatening infection.

The absolute neutrophil count and the relative proportions of neutrophils to lymphocytes vary with age. At birth, neutrophils predominate but rapidly decrease during the first few days of life. During infancy, neutrophils comprise approximately 20% to 30% of the circulating white blood cell population. By 5 years of age, neutrophil and lymphocyte counts are equal, and by puberty, adult proportions of approximately 70% are reached.

Neutropenia is defined by an absolute decrease in the absolute number of circulating mature (segmented) and band forms of neutrophils, which can be determined by calculating the absolute neutrophil count (ANC). The ANC is calculated by multiplying the total white blood cells (WBC) obtained from the complete blood count (CBC) by the combined percentage of segmented neutrophils and bands from the differential. The normal resting ANC in the general population ranges between 1500 and 8000 cells/cm3 for Caucasian children over 6 years of age, while 30% of African American children have an ANC as low as 1000 cells/μl. Based on the ANC, neutropenia can be classified as mild (ANC between 1000–1500 cells/μl), moderate (ANC between 500–1000 cells/μl), or severe (ANC < 500 cells/μl). Neutropenia is associated with an increased chance of developing infections. However, only patients with severe neutropenia are likely to develop life-threatening infections.

Patients with neutropenia are most frequently infected with endogenous flora, Staphylococcus aureus, and gram-negative organisms. Susceptibility to bacterial infections varies even in the presence of severe neutropenia. Some patients with chronic neutropenia syndromes with an ANC less than 200 cells do not develop life-threatening infections, while neutropenia induced by immunosuppressive drugs, particularly in conjunction with a malignancy, is associated with higher rates of severe infections, probably due to the additional loss of cellular immunity. Severe neutropenia is associated with skin and soft-tissue infections, gingivitis, stomatitis, pneumonia, and septicemia. However, isolated neutropenia is not associated with an increased risk for parasitic, viral, or fungal infections. Acute neutropenia arises when neutrophils are being employed and production is limited. Chronic neutropenia lasting months to years often evolves from impaired production or excessive splenic sequestration. Neutropenia may commonly occur from factors extrinsic to marrow myeloid cells or less frequently as an acquired disorder of myeloid and stem cells.

### Classification of Primary Neutropenias

The inherited disorders of bone marrow production, characterized by selective loss of neutrophil production without accompanying congenital anomalies, are diagnosed by clinical and laboratory features and by genetic testing (Table 441-1).1 Many of the inherited forms of congenital neutropenia fall into a category of ...

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