Microbiology, 2021

18 • Summary 759
SUMMARY
18.1 Overview of Specific Adaptive
Immunity
• Adaptive immunity is an acquired defense
against foreign pathogens that is characterized
by specificity and memory. The first exposure
to an antigen stimulates a primary response,
and subsequent exposures stimulate a faster
and strong secondary response.
• Adaptive immunity is a dual system involving
humoral immunity (antibodies produced by B
cells) and cellular immunity (T cells directed
against intracellular pathogens).
• Antigens, also called immunogens, are
molecules that activate adaptive immunity. A
single antigen possesses smaller epitopes, each
capable of inducing a specific adaptive immune
response.
• An antigen’s ability to stimulate an immune
response depends on several factors, including
its molecular class, molecular complexity, and
size.
• Antibodies (immunoglobulins) are Y-shaped
glycoproteins with two Fab sites for binding
antigens and an Fc portion involved in
complement activation and opsonization.
• The five classes of antibody are IgM, IgG, IgA,
IgE, and IgD, each differing in size,
arrangement, location within the body, and
function. The five primary functions of
antibodies are neutralization, opsonization,
agglutination, complement activation, and
antibody-dependent cell-mediated cytotoxicity
(ADCC).
18.2 Major Histocompatibility
Complexes and Antigen-Presenting
Cells
• Major histocompatibility complex (MHC) is a
collection of genes coding for glycoprotein
molecules expressed on the surface of all
nucleated cells.
• MHC I molecules are expressed on all nucleated
cells and are essential for presentation of
normal “self” antigens. Cells that become
infected by intracellular pathogens can present
foreign antigens on MHC I as well, marking the
infected cell for destruction.
• MHC II molecules are expressed only on the
surface of antigen-presenting cells
(macrophages, dendritic cells, and B cells).
Antigen presentation with MHC II is essential for
the activation of T cells.
• Antigen-presenting cells (APCs) primarily
ingest pathogens by phagocytosis, destroy them
in the phagolysosomes, process the protein
antigens, and select the most antigenic/
immunodominant epitopes with MHC II for
presentation to T cells.
• Cross-presentation is a mechanism of antigen
presentation and T-cell activation used by
dendritic cells not directly infected by the
pathogen; it involves phagocytosis of the
pathogen but presentation on MHC I rather than
MHC II.
18.3 T Lymphocytes and Cellular
Immunity
• Immature T lymphocytes are produced in the
red bone marrow and travel to the thymus for
maturation.
• Thymic selection is a three-step process of
negative and positive selection that determines
which T cells will mature and exit the thymus
into the peripheral bloodstream.
• Central tolerance involves negative selection of
self-reactive T cells in the thymus, and
peripheral tolerance involves anergy and
regulatory T cells that prevent self-reactive
immune responses and autoimmunity.
• The TCR is similar in structure to
immunoglobulins, but less complex. Millions of
unique epitope-binding TCRs are encoded
through a process of genetic rearrangement of
V, D, and J gene segments.
• T cells can be divided into three classes—helper
T cells, cytotoxic T cells, and regulatory T
cells—based on their expression of CD4 or CD8,
the MHC molecules with which they interact for
activation, and their respective functions.
• Activated helper T cells differentiate into T H 1,
T H 2, T H 17, or memory T cell subtypes.
Differentiation is directed by the specific
cytokines to which they are exposed. T H 1, T H 2,
and T H 17 perform different functions related to
stimulation of adaptive and innate immune
defenses. Memory T cells are long-lived cells
that can respond quickly to secondary
exposures.
• Once activated, cytotoxic T cells target and kill
cells infected with intracellular pathogens.
Killing requires recognition of specific pathogen
epitopes presented on the cell surface using
MHC I molecules. Killing is mediated by