Porth's Essentials of Pathophysiology, 4e

246

Hematopoietic Function

U N I T 3

Superficial cortex (B cell zone)

Nonneoplastic Disorders of White Blood Cells The number of leukocytes, or white blood cells, in the peripheral circulation normally ranges from 4500 to 10,500 cells/ μ L (4.5 to 10.5 × 10 9 /L). 6 The nonneoplas- tic disorders of white blood cells include a deficiency of leukocytes (i.e., leukopenia) or proliferation of excess white blood cells (i.e., leukocytosis). Neutropenia (Agranulocytosis) The term leukopenia describes a decrease in the absolute number of leukocytes in the blood. The disorder may affect any of the specific types of white blood cells, but most often affects the neutrophils, which are the pre- dominant type of granulocyte. Agranulocytosis denotes a virtual absence of neutrophils. In aplastic anemia, all of the myeloid stem cells are affected, resulting in ane- mia, thrombocytopenia, and agranulocytosis. Neutropenia refers specifically to an abnormally low number of neutrophils and is commonly defined as a circulating neutrophil count of less than 1500/ μ L. 4,7–11 It can be further graded as mild (1000 to 1500/ μ L), mod- erate (500 to 1000/ μ L), or severe (<500/ μ L) based on an absolute number of neutrophils circulating in the blood. Since the neutrophil protects against bacterial infec- tions, persons with neutropenia are prone to recurrent and sometimes severe bacterial infections. Individuals of African descent and some ethnic groups from the Middle East have lower neutrophil counts without predisposition to bacterial infection, a condition referred to as benign ethnic neutropenia . 11 ■■ The life span of white blood cells is relatively short so that constant renewal is necessary to maintain normal blood levels. Any conditions that decrease the availability of stem cells or hematopoietic growth factors produce a decrease in white blood cells. ■■ The lymphatic system consists of a network of lymphatic vessels, nodes, and tissues where B andT lymphocytes complete their differentiation. Lymph nodes, which are the site where many lymphomas originate, exhibit an outer cortex and an inner medulla.The cortex contains well-defined B-cell andT-cell domains.The B-cell–dependent superficial cortex consists of two types of follicles: immunologically inactive follicles, called primary follicles, and active follicles that contain germinal centers, called secondary follicles. Most of theT cells are contained in the paracortex, the area between the medullary and outer superficial cortices.

Secondary follicle

Primary follicle

Germinal center Mantle zone

Afferent lymphatic Trabecula Capsule

Paracortex (T cell zone)

Artery

Vein Efferent lymphatic FIGURE 11-4. Structures of normal lymph node.

The alimentary canal, respiratory passages, and genitourinary systems are guarded by accumulations of lymphoid tissue that are not enclosed in a capsule. This form of lymphoid tissue is called diffuse lymphoid tissue or mucosa-associated lymphoid tissue (MALT) because of its association with mucous membranes. Lymphocytes are found in the subepithelium of these tis- sues. Lymphomas can arise from either MALT or lymph node tissue.

SUMMARY CONCEPTS

■■ The hematopoietic system consists of the different types of blood cells generated from the pluripotent stem cells in the bone marrow. These stem cells differentiate into committed cell lines that develop into red blood cells, platelets, and leukocytes.The development of the different types of blood cells is supported by chemical messengers, called colony-stimulating factors, other growth factors, and chemical mediators. ■■ White blood cells (leukocytes) development begins with myeloid stem cells that develop into granulocyte and monocyte cell lines, and lymphoid stem cells that develop into the lymphocyte cell line.The immature precursor cells for each of the cell lines are called blast cells.The blast cells progress through various maturational stages before becoming mature granulocytes, monocytes, or lymphocytes.