Chapter 13
Innate and Adaptive Immunity
297
Humoral Immunity
Humoral immunity requires the presence of mature B lym-
phocytes capable of recognizing antigen and which can
ultimately mature into antibody-secreting plasma cells. The
ultimate response of the antigen–antibody complex formation
can take several forms including antigen–antibody complex
precipitation, agglutination of pathogens, neutralization of
toxins, phagocytosis or lysis of invading organisms, immune
cell activation, and complement activation.
Two separate but interrelated responses occur in the
development of humoral immunity: a primary and a secondary
response (Fig. 13.10). A
primary immune response
develops
when the body encounters the antigen for the first time. The
antigen comes in contact with various APCs including mac-
rophages, DCs, and B lymphocytes. The antigen is processed
by these cells in association with the MHC-II molecules on
the cells surface and then presented to the lymphocytes (
i.e.,
CD4
+
T-helper cells) to initiate the immune process. APCs
such as macrophages also secrete ILs, which are essential for
CD4
+
helper T cell activation.
31
The activated CD4
+
helper T
cells trigger B cells to proliferate and differentiate into clone
plasma cells that produce antibody. The primary immune
response takes 1 to 2 weeks, but once generated, detectable
antibody continues to rise for several more weeks even though
the infectious process has resolved. The
memory phase or
secondary immune response
occurs on subsequent exposure
to the antigen. During the secondary response, the rise in
antibody occurs sooner and reaches a higher level because of
available memory cells.
During the primary response, B lymphocytes proliferate
and differentiate into antibody-secreting plasma cells. A frac-
tion of the activated B cells do not undergo differentiation but
rather remain intact to form a pool of memory B lymphocytes
that then become available to efficiently respond to invasion
during subsequent exposure. Activated T cells can also gener-
ate primary and secondary cell-mediated immune responses
and the concurrent development of T memory cells.
The immunization process makes use of the primary and
secondary immune responses. The initial vaccination causes
production of both plasma cells and memory cells. The plasma
cells destroy the invading organism or toxin, and the mem-
ory cells provide defense against future exposure. “Booster”
immunizations produce an immediate antigen–antibody
response that simulates an immediate rise in antibody levels.
Current phase I clinical immunization trials for cancer treat-
ment show dense concentrations of CD4
+
and CD8
+
T lym-
phocytes and plasma cells in preexisting tumors after vaccina-
tion with irradiated malignant cells.
32
T Lymphocytes and Cellular
Immunity
T lymphocytes serve many functions in the immune system
including the activation of other T cells and B cells, control
of intracellular viral infections, rejection of foreign tissue
grafts, activation of autoimmune processes, and activation of
delayed hypersensitivity reactions. These processes make up
the body’s
cell-mediated
or
cellular immunity
. The effector
phase of cell-mediated immunity is carried out by T lympho-
cytes and macrophages.
T lymphocytes arise from lymphoid stem cells in the
bone marrow, but unlike B lymphocytes, they migrate to the
thymus gland to undergo the process of maturation. The thy-
mus gland is richly innervated and produces several peptide
Variable region
(heavy chain)
Constant region
(heavy chain)
Fab
Fc
Variable
region
(light chain)
Constant
region
(light chain)
Heavy
chain
Antigen
FIGURE 13.9
•
Schematic model of an immunoglobulin G (IgG)
molecule showing the constant and variable regions of the light and
heavy chains.
8
10
12
6
4
2
Weeks
Antigen
Antigen
Primary
response
Secondary
response
Activated
B cell
Activated
B cell
Memory
B cell
Naive
B cell
FIGURE 13.10
•
Primary and secondary or memory phases of the
humoral immune response to the same antigen.
