The Immune System: Innate and Adaptive Body Defenses
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Surface Barriers: Skin and Mucosae
Skin and mucous membranes constitute the ﬁrst line of defense.
Teir role is to prevent pathogens from entering the body.
Protective membranes line all body cavities and organs exposed
to the exterior.
Surface membranes provide mechanical barriers to pathogens.
Some have structural modiﬁcations and produce secretions
that enhance their defensive eﬀects: Te skin’s acidity, lysozyme,
mucus, keratin, and ciliated cells are examples.
IP Immune System; Topic: Innate Host Defenses, pp. 3–5.
Internal Innate Defenses: Cells and Chemicals
Te innate cellular and chemical defenses provide the body’s
second line of defense.
Phagocytes (macrophages, neutrophils, and the like) engulf and
destroy pathogens that breach epithelial barriers. Tis process is
facilitated when opsonins (antibodies or complement to which
the phagocyte’s receptors can bind) attach to the pathogen’s
surface. Te respiratory burst enhances cell killing.
Natural Killer Cells
Natural killer (NK) cells are large granular lymphocytes that act
nonspeciﬁcally to kill virus-infected and cancerous cells.
Inﬂammation: Tissue Response to Injury
Te inﬂammatory response prevents the spread of harmful
agents, disposes of pathogens and dead tissue cells, and promotes
healing. Exudate forms; protective leukocytes enter the area;
ﬁbrin walls oﬀ the area; and tissue repair occurs.
Te cardinal signs of inﬂammation are swelling, redness, heat,
and pain. Tese result from inﬂammatory chemicals that induce
vasodilation and make blood vessels more permeable. Impaired
function may be a ﬁFh cardinal sign.
Interferons are a group of related proteins synthesized by virus-
infected cells and certain immune cells that prevent viruses from
multiplying in other body cells.
Activation of complement (a group of plasma proteins) on the
membrane of a foreign cell promotes phagocytosis of that cell,
enhances inﬂammation, and sometimes causes lysis of the target cell.
±ever enhances the body’s ﬁght against pathogens by increasing
metabolism, which speeds up defensive actions and repair
processes, and by prompting the liver and spleen to sequester iron
and zinc needed for bacterial multiplication.
Immune System; Topic: Innate Host Defenses, pp. 6–23.
Te adaptive immune system recognizes something as foreign
and acts to immobilize, neutralize, or remove it. Te adaptive
immune response is antigen-speciﬁc, systemic, and has memory.
It provides the body’s third line of defense.
Immune System; Topic: Immune System Overview, p. 7.
Antigens are substances capable of generating an immune
Complete Antigens and Haptens
Complete antigens have both immunogenicity and reactivity.
Incomplete antigens or haptens must combine with a body
protein before becoming immunogenic.
Antigenic determinants are the portions of antigen molecules that
are recognized as foreign. Most antigens have many such sites.
Self-Antigens: MHC Proteins
Major histocompatibility complex (MHC) proteins are
membrane-bound glycoproteins that mark our cells as “self.”
Immune System; Topic: Immune System Overview, p. 8;
Topic: Cellular Immunity, pp. 5–8.
Cells of the Adaptive Immune System:
Lymphocytes arise from hematopoietic stem cells of the bone
marrow and are educated to develop immunocompetence and
self-tolerance. ² cells are educated in the thymus and provide
cellular immunity. B cells are educated in the bone marrow and
provide humoral immunity. Immunocompetence is signaled by
the appearance of antigen-speciﬁc receptors on the surface of the
lymphocyte. Immunocompetent lymphocytes seed the secondary
lymphoid organs, where the antigen encounter occurs, and
circulate between the blood, lymph, and lymphoid organs. When
naive lymphocytes encounter their antigen, clonal selection,
proliferation, and diﬀerentiation occur. Most of the clone
members become eﬀector cells, but some become memory cells.
In both B and ² lymphocytes, antigen receptor diversity is
generated by shuﬄing gene fragments.
Antigen-presenting cells (APCs) include dendritic cells,
macrophages, and B lymphocytes. Tey internalize antigens and
present antigenic determinants on their surfaces for recognition
by ² cells.