282 Anatomy, Histology, and Cell Biology
in the thymus. There are three types of T cells: cytotoxic T cells, T regula-
tory (Tr)cells, and helper T cells. Helper T cells possess the specific cell
membrane marker CD4, whereas the regulatory and cytotoxic subgroups
have CD8 on their cell surface. T cells require close contact with other
cells to perform their cell-mediated function. This is quite different from B
cells, where antibodies are secreted into the bloodstream. B cell gene
rearrangement occurs in the bone marrow during B cell education by a
similar process. It must be remembered that T cell receptors are antibody-
like heterodimers. Gene rearrangement in B and T cell education involves
similar V(D)J recombinations.
172.The answer is a.(Abbas, pp 47–52, 58–59. Alberts, pp 1396–1397,
1412–1420. Junqueira, pp 258–261.)Fragments of antigen associated with
class II MHC glycoproteins are recognized by helper T cells. Activation of
helper T cells is required as an early step in the immune response. For B
and Tccells to respond to most antigens, helper T cells are required. How-
ever, in the case of some bacterial polysaccharides, B cells respond to the
antigens in the absence of helper T cells. The primary cell type for expres-
sion of class II MHC is the macrophage or dendritic cell (antigen-presenting
cells, APCs), but thymic epithelial cells and B cells can also present antigen
under appropriate conditions.
173.The answer is c.(Abbas, pp 91–92. Alberts, pp 1394–1406.)Cyto-
toxic T cells possess the CD8 cell surface marker. CD8+T cells recognize
foreign antigen in association with class I MHC molecules. Cytotoxic T
cells are effective in killing virus-infected cells because those cells express
fragments of viral proteins combined with MHC I molecules on their sur-
faces. In contrast, helper T cells (CD4+) recognize antigen in association
with class II MHC molecules. MHC Class I is present on the surface of most
cells, whereas antigen-presenting cells (including B lymphocytes) and
thymic epithelial cells possess MHC Class II.
174.The answer is c.(Abbas, pp 93–94. Kindt, pp 68, 307–312. Junqueira,
pp 260–262.)Interleukin-2 (IL-2) is produced by helper T cells (cell surface
marker CD4). IL-2 stimulates proliferation of T and B cells. IL-2 is a
cytokine, that affects the proliferation and differentiation of other cells. Antigen-
presenting cells make interleukin (IL)-1 with helper T cells as the primary