Immunologic Aspects of Organ Transplantation

Susan Smith MN, PhD


June 17, 2002


Two concepts are important to understanding how leukocytes become immunocompetent cells: proliferation and differentiation (Figure 6). Proliferation is the production of many cells from a single cell through repeated mitosis of daughter cells. Differentiation is the process of cell maturation. Through differentiation, cells acquire their ultimate functions and the protein characteristics required to perform those functions. In general, the more immature a cell is, the bigger it is and the greater its ability to proliferate. As a cell differentiates, it becomes smaller, loses its ability to proliferate, and focuses its energy instead on performing its function.

Figure 6.

Cell proliferation and differentiation.

Leukocytes begin as pluripotent hematopoietic stem cells in the bone marrow and develop along 1 of 2 major lineages: the myeloid lineage or the lymphoid lineage megakaryocytes, eosinophils, neutrophils, basophils/mast cells, and monocytes/macrophages. Auxiliary myeloid cells include megakaryocytes (precursors to platelets) and mast cells. Platelets are not true progeny of megakaryocytes. Instead, they are fragments of megakaryocytes.

The WBC Count

Normally, there are approximately 4000 to 11,000 leukocytes/mm3 of human blood, which equals 4 x 106 to 11 x 106 WBCs/L of blood. By contrast, there are normally about 5 x 109 RBCs/L of blood, fully 3 orders of magnitude greater than the number of leukocytes. However, in spite of their relatively small numbers, WBCs are crucial to immune defense.

The WBC Differential Count

The WBC differential count provides a snapshot of granulocytes, lymphocytes, and monocytes in the peripheral blood. The WBC total and differential counts are important diagnostic tools that aid in monitoring clinical status and the effectiveness of therapeutic interventions. Currently, routine blood cell counts are rarely performed under the microscope. Instead, automated electronic counters are used; these are designed to detect the differences in cell nuclear and cytoplasmic morphology that distinguish 1 type of leukocyte from another.

Table 2 depicts a typical WBC differential count for a healthy adult. About 60% to 80% of WBCs are neutrophils, 20% to 40% are lymphocytes, and 2% to 10% are monocytes. About 80% of the lymphocytes are T cells; the remaining 20% are B cells, but up to 5% may be NK cells.

In general, very few basophils or eosinophils circulate in the blood. WBCs of every kind marginate on the walls of the blood vessels, ready to enter the tissues in case of bleeding, usually accompanied by the entry of foreign antigen under the skin. These marginated leukocytes are not evident in blood sampled for WBC total or differential counts.

The Myeloid Lineage

Myeloid cells make up the backbone of the natural or innate defense system. Myeloid leukocytes can be classified into 2 major groups: granulocytes and monocytes. The major function of both is phagocytosis. Phagocytosis, which means "cell eating," is the first event of host defense when a foreign material enters the body. This process is carried out by a network of highly mobile phagocytes in the blood and other tissues that is collectively referred to as the reticuloendothelial system (RES). Phagocytes have surface receptors that allow them to seek out nonspecific foreign organisms, engulf them, and ultimately destroy them. Phagocytosis is the process by which excess antigen and dead cells are removed from the body (Figure 7). Phagocytosis is also essential in the initiation of cellular and humoral immune responses by B cells and T cells. There are large numbers of macrophages strategically located ( Table 4 ) in the liver, spleen, lungs, kidney, and lymph nodes, where they act as filters to remove and destroy infectious organisms and bodily debris (eg, senescent RBCs).

Figure 7.

Bacterial degradation by neutrophils.

Granulocytes. Granulocytes, commonly referred to as polymorphonuclear granulocytes (PMNs), "polymorphs," or "polys," are produced in the bone marrow at the rate of approximately 80 million per day, and their average life span is about 2 to 3 days. Sixty percent to 70% of all leukocytes are PMNs. These cells are called "polymorphs" because their nuclei are multilobed; they are called granulocytes because they contain intracellular granules. Their intracellular granules contain packets of hydrolytic enzymes (lysozymes), making the cells cytotoxic to foreign organisms, particularly bacteria (Figure 7). Lysozomes are encased in portions of membrane within the neutrophil until the cell encounters a bacterium. The lysozome fuses with the portion of neutrophil cell membrane that has trapped the bacterium and releases its contents, resulting in digestion and destruction of the bacterium. Granulocytes are classified into 3 more distinct types: neutrophils, eosinophils, and basophils, according to the histologic staining reactions of the intracellular granules.

Neutrophils are the most abundant cells in the bone marrow and blood, comprising about 90% of all PMNs or granulocytes. Three forms of neutrophils can be identified in the peripheral blood: segmented neutrophils or "segs," "bands," and metamyelocytes. Segmented neutrophils are fully mature, bands are slightly immature, and metamyelocytes are completely immature neutrophils. Neutrophils circulate in the blood for only about 12 hours. Then they move into the tissues, where they live for only a few days.

Neutrophils are strongly phagocytic; that is, they ingest microorganisms or other cells and foreign particles and digest the ingested material within their phagocytic vacuoles. There is an increased demand for neutrophils with infection. The bone marrow responds by releasing more neutrophils into the circulation. Immature forms of neutrophils are released along with the mature cells when the demand exceeds the supply. As a result, the percentage of bands in the peripheral blood is increased. This condition, referred to as a "shift to the left," indicates acute inflammation. In more serious conditions, metamyelocytes will also appear in increased numbers in the peripheral blood. The normal neutrophil count in the adult is 1000/mm3 to 6000/mm3 blood, or approximately 65% of circulating WBCs or the differential WBC count. Bands normally number about 600/mm3 of blood, or approximately 0% to 5% of the differential WBC count.

Eosinophils are weakly phagocytic (much less so than neutrophils) granulocytes with a bi-lobed nucleus that appear in increased numbers in the circulation, specifically during parasitic infections (ie, worm infestation) and allergic hypersensitivity reactions. Eosinophils degranulate on antigenic stimulation and kill organisms extracellularly. The normal eosinophil count is about 200/mm3 of blood, or 2% to 5% of the differential WBC count.

Basophils are granulocytes with a multilobed nucleus that are responsible for anaphylactoid reactions to allergens. Like eosinophils, basophils are capable of releasing their cytotoxic granules when stimulated by certain antigens to effect extracellular killing. Basophils are morphologically identical to mast cells, but can be differentiated from mast cells in that basophils are blood borne and mast cells reside in tissues outside the circulation. In other words, when a basophil migrates out of the circulation to reside in tissue, it becomes a mast cell. The normal basophil count is about 100/mm3 of blood, or about 0.2% of the differential WBC count. Polymorphonuclear granulocytes can be differentiated from monocytes by their multilobed nuclei and many intracellular granules.

Monocytes. Monocytes are mononuclear cells that do not contain cytotoxic granules. They do, however, release the prostaglandin PGE2, which is a mediator of the inflammatory response. Monocytes circulate in the blood for approximately 24 hours and then move into the tissues, where they develop into macrophages. Macrophages can live in the tissues for many months as phagocytes. Macrophages are nonspecific accessory cells that play a role in primary host defense, control neoplasia, scavenge damaged or dying cells, and interact with lymphocytes to facilitate cellular and humoral immunity. The normal monocyte count is about 200/mm3 to 1000/mm3 of blood, or about 5% of the differential WBC count.

The Lymphoid Lineage

Common lymphoid progenitor cells have the capacity to differentiate into either B lymphocytes (B cells) or T lymphocytes (T cells), depending on the microenvironment in which they live and develop. By virtue of antigen-specific receptors located on their membranes, these cells have the ability to distinguish 1 antigen from another. Lymphocytes are discussed in more detail in the "Acquired Immune Responses" section.