Chapter 13
Innate and Adaptive Immunity
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Undifferentiated, immature lymphocytes migrate to lymphoid
tissues, where they develop into distinct types of mature lym-
phocytes (Fig. 13.5). The T lymphocytes first migrate to the
thymus gland where they divide rapidly and develop extensive
diversity in their ability to react against different antigens.
26
Each T lymphocyte develops specificity against a specific anti-
gen. Once this differentiation occurs, the lymphocytes leave
the thymus gland and migrate via the bloodstream to periph-
eral lymphoid tissue. At this time, they have been prepro-
grammed not to attack the body’s own issues. Unfortunately,
in many autoimmune diseases it is believed that this process
goes astray. The
B lymphocytes
mature primarily in the bone
marrow and are essential for
humoral
, or
antibody-mediated
,
immunity
. Unlike the T lymphocytes, where the entire cell is
involved in the immune response, B lymphocytes secrete anti-
bodies, which then act as the reactive agent in the immune
process. Therefore, the lymphocytes are distinguished by their
function and response to antigen, their cell membrane mole-
cules and receptors, their types of secreted proteins, and their
tissue location. High concentrations of mature lymphocytes
are found in the lymph tissue throughout the body including
the lymph nodes, spleen, skin, and mucosal tissues.
T and B lymphocytes possess all of the processes neces-
sary for the adaptive immune response—specificity, diversity,
memory, and self–nonself recognition. When antigens come
in contact with the lymphocytes in the lymphoid tissues of the
is responsible for initiating an allergic reaction. An allergic
response to the antibiotic penicillin is an example of a medi-
cally important reaction due to hapten–carrier complexes. The
penicillin molecule is very small (350 Da) and usually nonan-
tigenic. However, in susceptible people it can complex with
carrier proteins in the body, which are then recognized as “for-
eign” and capable of initiating an antigen–antibody reaction.
Cells of Adaptive Immunity
The principal cells of the adaptive immune system are the
lymphocytes, APCs, and effector cells.
Lymphocytes
Lymphocytes make up approximately 36% of the total white
cell count and are the primary cells of the adaptive immune
response. They arise from the lymphoid stem cell line in the
bone marrow and differentiate into two distinct but inter-
related cell types: the B lymphocytes and T lymphocytes.
B lymphocytes are responsible for forming the antibodies that
provide humoral immunity, whereas T lymphocytes provide
cell-mediated immunity. T and B lymphocytes are unique in
that they are the only cells in the body capable of recognizing
specific antigens present on the surfaces of microbial agents
and other pathogens. As a result, adaptive immune processes
are organism specific and possess the capacity for memory.
The recognition of specific surface antigens by lym-
phocytes is made possible because of the presence of specific
receptors or antibodies on the surface of B and T lymphocytes.
Scientists have been able to identify these specific proteins and
correlate them with a specific cellular function. This has lead
to the development of a classification system for these surface
molecules known as the “cluster of differentiation” (CD). The
nomenclature for the surface proteins utilizes the letters “CD”
followed by a number that specifies the surface proteins that
define a particular cell type or stage of cell differentiation and are
recognized by a cluster or group of antibodies. The utilization of
this nomenclature has spread to other immune cells and cyto-
kines all of which contribute to the acquired immune response.
Leukocytes involved in the innate immune response,
such as macrophages and DCs, also play a key role in adap-
tive immunity because they function as APCs. They are capa-
ble of processing complex antigens into epitopes, which are
then displayed on their cell membranes in order to activate the
appropriate lymphocytes. Functionally, there are two types of
immune cells: regulatory cells and effector cells. The
regula-
tory cells
assist in orchestrating and controlling the immune
response, while effector cells carry out the elimination of
the antigen (microbial, nonmicrobial, or toxin). In the body,
helper T lymphocytes activate other lymphocytes and phago-
cytes, while regulatory T cells keep these cells in check so that
an exaggerated immune response does not occur. Cytotoxic
T lymphocytes, macrophages, and other leukocytes function
as effector cells in different immune responses.
While T and B lymphocytes are generated from lymphoid
stem cells in the bone marrow, they do not stay there to mature.
Thymus
Bone marrow
or fetal liver
Pluripotent
stem cell
Lymphocyte
stem cell
B cell
T cell
Lymphoid tissue (lymph nodes, spleen,
mucosal tissue, blood, and lymph)
FIGURE 13.5
•
Pathway for T- and B-cell differentiation.
