Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1308 Chapter 24: The Innate and Adaptive Immune Systems
adenoid
thymus
Peyer's patches in
small intestine
tonsil
lymphatic vessels
lymph nodes
spleen
Figure 24–12 Human lymphoid organs.
Lymphocytes develop from lymphoid
progenitor cells in the thymus and bone
marrow (yellow), which are therefore called
central (or primary) lymphoid organs.
The newly formed lymphocytes migrate
from these primary organs to peripheral
(or secondary) lymphoid organs, where
they can react with foreign antigen. Only
some of the peripheral lymphoid organs
(blue) and lymphatic vessels (green) are
shown; many lymphocytes, for example,
are found in the skin and respiratory tract.
As we discuss later, the lymphatic vessels
ultimately empty into the bloodstream (not
shown).
appendix
bone marrow
B Cells Develop in the Bone Marrow, T Cells in the Thymus
There are about 2 × 10 12 lymphocytes in the human body, making the immune
system comparable in cell mass to the liver or the brain. They occur in large numbers
in the blood and lymph (the colorless fluid in the lymphatic vessels, which
connect the lymph nodes in the MBoC6 body m25.03/24.13
to each other and to the bloodstream). They
are also concentrated in lymphoid organs, such as the thymus, lymph nodes, and
spleen (Figure 24–12), and many are also found in other organs, including skin,
lung, and gut.
T cells and B cells derive their names from the organs in which they develop:
T cells develop in the thymus, and B cells, in adult mammals, develop in the bone
marrow. Both types of cells develop from lymphoid progenitor cells that are produced
from multipotent hematopoietic stem cells, which are found mainly in the
bone marrow (Figure 24–13). The hematopoietic stem cells give rise to more than
just lymphocytes: as discussed in Chapter 22, they produce all of the cells of the
bone
marrow
thymus
peripheral
lymphoid organs
lymphoid
progenitor
cell
hematopoietic
stem cells
lymphoid
progenitor
cell
developing
B cell
thymocyte
T cell
antigen
B cell
T-CELL-
MEDIATED
IMMUNE
RESPONSE
ANTIBODY
RESPONSE
Figure 24–13 The development of
B and T cells. The central lymphoid
organs, where lymphocytes develop from
lymphoid progenitor cells, are labeled in
yellow boxes. The lymphoid progenitor cells
develop from multipotent hematopoietic
stem cells in the bone marrow. Some
lymphoid progenitor cells develop locally
in the bone marrow into immature B cells,
while others migrate to the thymus (via
the bloodstream) where they develop into
thymocytes (developing T cells). Foreign
antigens activate B cells and T cells mainly
in peripheral lymphoid organs, such as
lymph nodes or the spleen.