CHAPTER 10
Lymphoid System
183
Figure 10-2.
A representation of types of lymphocytes.
B lymphocytes (B cells)
,
T lymphocytes (T cells)
, and
null cells
are three major cell types in the immune system. Each of these
cells originates from precursor cells in the bone marrow. B and T lymphocytes are the main cell types located in lymphoid organs.
(1)
B cells
mature and become
naive
(
virgin
) B cells (immunocompetent cells that have not been previously exposed to foreign
antigen) in the bone marrow; they migrate to secondary lymph organs and may meet with antigens. B cells that become activated
by exposure to antigens differentiate into memory B cells and effector B cells (plasma cells). (2)
T cells
differentiate from pro–T
lymphocytes, which have migrated from the bone marrow into the thymus through the circulatory system.
Thymocytes
(developing
lymphocytes) differentiate to
naive
(
virgin
) T cells in the thymus and then migrate to secondary lymphoid organs where they may be
activated by exposure to foreign antigens. Activated T cells can differentiate into both
memory T cells
and
effector T cells
. Effector T
cells include
helper T cells
,
cytotoxic T cells
, and
regulatory
(
suppressor
)
T cells
. B and T cells share some common features. Each B
and T cell is programmed to respond to a particular antigenic determinant. Each naive B cell or T cell is relatively short lived unless
it becomes activated by contact with the antigen it recognizes. Both types give rise to both memory cells and effector cells if they
interact with an antigen (“antigen dependent”). Both B and T cells reside in speciF
c regions in secondary lymphoid organs. However,
there are some important differences between B and T cells. B-cell antigen recognition is mediated by Ig molecules in their surface
membranes, whereas T-cell antigen recognition is mediated by the
T-cell receptor
(
TCR
), and activation requires presentation of
the antigen in association with an
MHC
molecule on the surface of another cell. ±inally, activated B cells function by differentiating
into antibody-secreting plasma cells (humoral immune response), whereas activated T cells can differentiate into several functional
forms: helper T cells, cytotoxic T cells, or suppressor T cells (cell-mediated immune responses). (3)
Null cells
are described in detail
in the introduction.
Null cells
(bone marrow,
circulation)
Pro–T lymphocytes
(bone marrow)
Pro–B lymphocytes
(bone marrow, fetal liver)
Hematopoietic stem cells
(bone marrow)
Virgin (inactive)
B lymphocytes
(circulation to lymphoid organs)
T lymphoblasts
(circulation to thymus)
Pluripotential
hemopoietic
stem cells (PHSCs)
Natural killer
(NK) cells
Plasma cells
(lymphoid organs to
connective tissue)
Virgin
T cells
(inactive)
Effector
T cells
Memory B cells
(lymphoid organs to
circulation)
Memory
T cells
Helper T cells
(Th0, Th1, Th2)
Cytotoxic
T cells
Regulatory
(suppressor)
T cells
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