Adaptive Immunity
Adaptive immunity (also called
acquired immunity
) refers
to immunity that is acquired through previous exposure to
infectious and other foreign agents. A defining character-
istic of adaptive immunity is the ability not only to distin-
guish self from nonself but to recognize and destroy specific
foreign agents based on their distinct antigenic properties.
The components of the adaptive immune system are the T
and B lymphocytes and their products. There are two types
of adaptive immune responses, humoral and cell-mediated
immunity, that function to eliminate different types of
microbes.
Humoral immunity is mediated by the B lymphocytes
(B cells) and is the principal defense against extracellular
microbes and their toxins. The B cells differentiate into
antibody-secreting plasma cells. The circulating antibodies
then interact with and destroy the microbes that are present
in the blood or mucosal surfaces.
Cell-mediated, or cellular, immunity is mediated by
the cytotoxic T lymphocytes (T cells) and functions in
the elimination of intracellular pathogens (
e.g.,
viruses).
T cells develop receptors that recognize the viral peptides
displayed on the surface of infected cells and then signal
destruction of the infected cells.
Extracellular
pathogen
Cell with intracellular
pathogen being destroyed
by cytotoxic T cell
B cell
Plasma
cell
Antibody
Cytotoxic
T cell
Cell death
MHC-I
with viral
epitope
TCR
Humoral immunity (B lymphocytes)
Lymphocyte
Cell-mediated immunity (T lymphocytes)
IN SUMMARY
Immunity is the body’s defense against disease and invad-
ing microorganisms. Immune mechanisms can be divided
into two types: innate and adaptive immunity. Innate
immunity is the first line of defense and can distinguish
between self and nonself through the recognition of cellu-
lar patterns on foreign substances and microbes. Adaptive
immunity is part of the second line of defense and involves
both humoral and cellular mechanisms that respond to
cell-specific substances known as antigens. The adaptive
immune response is capable of amplifying and sustaining
its responses, of distinguishing self from nonself, and
finally of memory in that it can recognize the antigen on
repeat exposure in order to quickly produce a heightened
response on subsequent encounters with the same microor-
Chapter 13
Innate and Adaptive Immunity
281
ganism. The innate and adaptive immune responses work
in concert with one another to ensure that the homeostasis
is maintained.
Although cells of both the innate and adaptive immune
systems communicate critical information about the
invading microbe or pathogen by cell-to-cell contact,
many interactions and cellular responses depend on the
secretion of chemical mediators in the form of cytokines,
chemokines, and CSFs. Cytokines are soluble proteins
secreted by cells of both the innate and adaptive immune
systems that mediate many of the functions of these cells.
Chemokines are cytokines that stimulate the migration
and activation of various immune and inflammatory cells.
CSFs stimulate the growth and differentiation of bone
marrow progenitors of immune cells and play a key role
in hematopoiesis.
