The capability of responding to immunologic stimuli rests mainly with lymphoid cells. During embryonic development, blood cell precursors originate mainly in the fetal liver and yolk sac; in postnatal life, the stem cells reside in the bone marrow. Stem cells differentiate into cells of the erythroid, myeloid, or lymphoid series. The latter evolve into two main lymphocyte populations: T cells and B cells (Figure 58–1 and Table 58–1). The formation of T cells and B cells from stem cells is enhanced by interleukin-7 (IL-7) produced by the stromal cells of the thymus and bone marrow, respectively.
Origin of T and B cells. Stem cells in the bone marrow (or fetal liver) are the precursors of both T and B lymphocytes. Stem cells differentiate into T cells in the thymus, whereas they differentiate into B cells in the bone marrow. Within the thymus, T cells become either CD4-positive (helper) cells or CD8-positive (cytotoxic) cells. B cells can differentiate into plasma cells that produce large amounts of antibodies (immunoglobulins). Dotted lines indicate interactions mediated by interleukins. (Reproduced with permission from Brooks GF et al. Medical Microbiology. 20th ed. Originally published by Appleton & Lange. Copyright 1995 McGraw-Hill.)
TABLE 58–1Comparison of T Cells and B Cells |Favorite Table|Download (.pdf) TABLE 58–1 Comparison of T Cells and B Cells
|Feature ||T Cells ||B Cells |
|Antigen receptors on surface ||Yes ||Yes |
|Antigen receptor recognizes only processed peptides in association with MHC protein ||Yes ||No |
|Antigen receptor recognizes whole, unprocessed proteins and has no requirement for presentation by MHC protein ||No ||Yes |
|IgM on surface ||No ||Yes |
|CD3 proteins on surface ||Yes ||No |
|Clonal expansion after contact with specific antigen ||Yes ||Yes |
|Immunoglobulin synthesis ||No ||Yes |
|Regulator of antibody synthesis ||Yes ||No |
|IL-2, IL-4, IL-5, and gamma interferon synthesis ||Yes ||No |
|Effector of cell-mediated immunity ||Yes ||No |
|Maturation in thymus ||Yes ||No |
|Maturation in bone marrow ||No ||Yes |
The ratio of T cells to B cells is approximately 3:1. Figure 58–1 describes the origin of B cells and the two types of T cells: helper T cells and cytotoxic T cells. Table 58–1 compares various important features of B cells and T cells. These features will be described in detail later in the chapter.
T-cell precursors differentiate into immunocompetent T cells within the thymus. Prior to entering the thymus, stem cells lack antigen receptors and lack CD3, CD4, and CD8 proteins on their surface. During passage through the thymus, they differentiate into T cells that can express both antigen receptors and the various CD proteins. The stem cells, which initially express neither CD4 nor CD8 (double-negatives), first ...