which are complementary to addressins on the postcapillary high endothe-
lial venules (HEVs) and explains the specificity of lymphocyte homing. The
cells that line the HEVs permit the selective passage of lymphocytes by dia-
pedesis through the intercellular junctions. Lymphocytes have specific
homing receptors on their cell surfaces that provide entry for mucosal (ver-
sus lymph node) seeding. High endothelial venules (HEVs) provide a
mechanism for lymphocytes to leave the bloodstream and enter specific
areas of the lymph nodes. HEVs are also found in Peyer’s patches and dur-
ing inflammation of tissues (e.g., the synovium in rheumatoid arthritis).
Under normal conditions, HEVs are found in the T-dependent areas, that
is, the deep cortex (paracortex) of the lymph nodes and the interfollicular
regions of the Peyer’s patches. T cells home to T-dependent areas of the
lymph nodes, spleen, and Peyer’s patches. The circulation and recirculation
of lymphocytes is a constant process that allows lymphocytes to continu-
ously monitor the presence of antigen. The circulation process also allows
augmentation of the immune response to infection. Plasma cells do not
enter the bloodstream under normal conditions, but secrete antibodies into
the circulation from the medulla of the lymph nodes or the marginal zone
of the spleen. Lymphocytes and other cells (e.g., monocytes and neu-
trophils) that leave the blood never pass through the endothelial cells.
In the histologic section of a lymph node, there is a distinctive cortex
and medulla with a connective tissue capsule. The organ possesses the clas-
sic bean shape with a hilus (marked by an asterisk in the figure). Afferent
lymphatics enter the lymph node on the convex side, and lymph percolates
through the subcapsular, cortical, and medullary sinuses. The medullary
sinuses converge on the hilus, where the efferent lymphatic vessel drains
the node. The hilus also contains an artery and a vein.
181.The answer is b.(Alberts, pp 1366, 1394. Junqueira, pp 93–97. Kindt,
pp 65–68, 263–264.)Macrophages are a group of monocyte-derived phago-
cytic cells that present antigen and synthesize IL-1. Macrophages arise from
the bone marrow (monocytes) and include the Kupffer cells of the liver,
Langerhans cells of the skin, and microglia of the central nervous system.
Antigen presentation is the process by which macrophages, dendritic cells,
and B cells phagocytose antigen and partially degrade the antigen in the
endosomal system. Certain portions of the antigen are returned to the cell
surface in combination with type II MHC as a complex. IL-1 activates the
helper T cell. Although macrophages may be required for the differentiation
Lymphoid System and Cellular Immunology Answers 287