USA Surgery Today

Hematopoiesis in the human embryo and fetus can be divided into three overlapping phases: mesoblastic, hepatic, and myeloid (1). The earliest cells of erythroid lineage have been detected at day 14 of gestation within the blood islands of the yolk sac. By day 35, erythropoiesis occurs in the liver, and granulopoiesis and thrombocytopoiesis follow. Until 6 months gestation, the liver is the major site of hematopoiesis. The myeloid phase starts in the fourth month, when hematopoiesis, including monocyte and lymphocyte production, begins in the bone marrow. The bone marrow becomes the major site of hematopoiesis. Through early childhood, most of the bone marrow space is occupied by red (hematopoietically active) marrow. With age, the distal marrow spaces slowly fill with fat.

Two of the main functions of T cells are (1) to provide cytotoxic activity against facultative intracellular pathogens (mycobacteria, fungi) and virally infected cells or tumor cells, and (2) to provide cytokines (Table 15-2) to help B cells make antibodies.

For the T cell to carry out these functions, it first must bind to the target cell or to the antigen-presenting cell (APC), respectively. For high-affinity binding of T cells to target cells or APCs, several molecules on T cells in addition to TCRs bind to molecules on their respective target cells (Fig. 15-2).

The dose of bacterial contamination is a key determinant of infection. Quantitatively, if a surgical site is contaminated with more than 10⁵ microorganisms per gram of tissue, then the risk of SSI is markedly increased (4). The dose of microorganisms required to produce infection may be considerably lower in the presence of foreign material (i.e., 100 staphylococci per gram of tissue introduced on silk suture) (5). The number of organisms required to produce clinical infection predictably decreases in states of diminished host resistance.

The virulence of a microorganism refers to its ability to invade, damage, or survive in host tissue. Some gram-negative bacteria elaborate endotoxin that causes no local injury, but stimulates cytokine production. Cytokines trigger the systemic inflammatory response syndrome (SIRS), which may lead to organ failure. Other bacteria possess cell surface polysaccharide capsules that inhibit phagocytosis, an early host defense to microbial contamination.