Fc epsilon RII, also known as CD23. Immunoglobulin-producing cells are
found in the germinal centers of the white pulp of the spleen and other sec-
ondary lymphoid organs (i.e., germinal centers in the cortex of the spleen and
in the tonsils and lymphoid follicles in the MALT and SALT). B-lymphocytes
can be identified by the presence of immunoglobulin on their surface mem-
branes and differentiate into antibody-secreting plasma cells under the appro-
priate conditions. T lymphocytes, on the other hand, do not have readily
detectable cell membrane immunoglobulin. The thymus is the site of T cell
education and the deep cortex of the lymph node and the periarteriolar lym-
phoid sheath (PALS) are T-dependent regions (answers a and e).
177.The answer is c.(Alberts, pp 1375–1384. Kindt, pp 128–130. Abbas,
pp 134, 136, 156.)Immunoglobulin switching normally occurs in the ger-
minal centers during the maturation of B cells. Synthesis of B cell antibody
begins as IgM inserted into the cell membrane and then switches to mem-
brane-bound IgM and IgD. After antigen stimulation, a switch to surface
IgM, IgA, IgG, or IgE occurs, and those antibodies are secreted. Most anti-
body production occurs in the germinal centers of the lymph nodes, ton-
sils, and spleen. It occurs to a lesser extent in the bone marrow (answer a),
but the bone marrow functions in the education of B cells as well as repre-
senting the major site of hematopoiesis in the adult. The thymus is respon-
sible for the education of T cells (answer d).The splenic red pulp is the
site of red blood cell breakdown (answer e).
Recognition of antigen by B cells is accomplished by the expression of
IgM molecules on the cell surface. Some investigators use the term pre-B
cell, or virgin B cell, to distinguish those B cells that have not yet synthe-
sized IgM from those that have synthesized and inserted IgM into their cell
membranes. IgD, which is produced later by maturing B cells, also serves
as an antigen receptor. Immunoglobulin switching does not occur in
peripheral blood (answer b).
178.The answer is c.(Alberts, pp 1410–1418. Kindt, p 393. Junqueira,
pp 260–262.)In a tuberculin skin test, T cell proliferation is increased by
secretion of interleukins. An extract of tuberculin (an antigen of lipopro-
tein composition obtained from the tubercle bacillus) is injected into the
skin of a person who has had tuberculosis or has been immunized against
tuberculosis. Memory helper T cells react to the tuberculin and secrete IL-2,
which upregulates IL-2 receptors. IL-2 binding to IL-2 receptors on the
same cell is an example of autocrine regulation in which a cell secretes a
Lymphoid System and Cellular Immunology Answers 285