CHAPTER
15
Innate Immunity
Chapter Outline
An Overview of the Body’s Defenses (p. 444)
The Body’s First Line of Defense (pp. 444–448)
The Role of Skin in Innate Immunity
The Role of Mucous Membranes in Innate Immunity
The Body’s Second Line of Defense (pp. 448–462)
Defense Components of Blood
Phagocytosis
Nonphagocytic Killing
Nonspecific Chemical Defenses Against Pathogens
Interactive Microbiology: Complement
Inflammation
Fever
Chapter Summary
An Overview of the Body’s Defenses (p. 444)
Because the cells and certain basic physiological processes of humans are incompatible with
those of most plant and animal pathogens, humans have species resistance to these pathogens.
Nevertheless, we are confronted every day with pathogens that do cause disease in humans. It is
convenient to cluster the body structures, cells, and chemicals that act against pathogens into
three main lines of defense:
2. The second line is also our innate immunity. It is internal and is composed of protective cells,
bloodborne chemicals, and processes that inactivate or kill invaders.
ter 16.
Chapter 15 Innate Immunity
The Body’s First Line of Defense (pp. 444–448)
The skin and mucous membranes present a formidable barrier to the entrance of microorgan-
isms.
The Role of Skin in Innate Immunity
The skin is composed of an outer epidermis, which is in turn composed of multiple layers of
tightly packed cells that constitute a barrier to most bacteria, fungi, and viruses. In addition, mi-
croorganisms attached to the epidermis are routinely sloughed off with the flakes of dead skin
cells. The epidermis also contains phagocytic cells called dendritic cells. Their slender process-
es form an almost continuous network to intercept invaders.
The Role of Mucous Membranes in Innate Immunity
Mucous membranes line the lumens of the respiratory, urinary, digestive, and reproductive
tracts. The epithelial cells of the outermost layer, or epithelium, are tightly packed but form only
a thin layer. They play key roles in the diffusion of nutrients and oxygen and in the elimination
The Role of the Lacrimal Apparatus in Innate Immunity
The lacrimal apparatus of the eye produces tears, which wash away pathogens. Lacrimal glands
secrete tears into ducts, which empty into the nasal cavity. Tears also contain lysozyme to de-
stroy bacteria.
The Role of the Microbiome in Innate Immunity
The skin and mucous membranes are normally home to a variety of protozoa, fungi, bacteria,
and viruses collectively referred to as the microbiome. The normal microbiota play an im-
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
– Blocking attachment of pathogens by attaching to human cells.
– Stimulating the body’s second line of defense.
Other First-Line Defenses
Antimicrobial peptides on the skin and in mucous membranes act against microorganisms. They
are triggered by sugars and proteins on the surface of microbes. Defensins may act on only cer-
tain microbes or on a broad range, and damage them by various means. Antimicrobial peptides
The Body’s Second Line of Defense (pp. 448–462)
The second line of defense includes no barriers. It is composed of cells such as phagocytes, an-
timicrobial chemicals such as complement and interferons, and the processes of inflammation
and fever.
Defense Components of Blood
Blood is composed of formed elements (cells and parts of cells) within a fluid called plasma.
Plasma is mostly water containing electrolytes, dissolved gases, nutrients, and protective pro-
teins such as clotting factors, complement proteins, and antibodies. Serum is plasma without
Defensive Blood Cells: Leukocytes
Cells and cell fragments in the plasma are called formed elements. Hemaptopoiesis refers to the
process by which blood stem cells in the bone marrow differentiate to form three types of
formed elements of the blood: erythrocytes fired blood cells), leukocytes (white blood cells),
and platelets (cell fragments involved in blood clotting). Leukocytes are divided into two
groups according to their appearance in stained blood smears.
Chapter 15 Innate Immunity
chemicals. Agranulocytes do not appear to have granules when viewed under light microscopy;
however, granules become visible with electron microscopy. They are of two types: lympho-
Phagocytosis
Cells of the body capable of phagocytosis are called phagocytes. Phagocytes use phagocytosis
to rid the body of pathogens that have evaded the body’s first line of defense. Phagocytosis is a
continuous process that can be divided into five steps:
1. Chemotaxis. Chemotaxis is movement of a cell either toward or away from a chemical
2. Adhesion. The phagocytes attach to pathogens via a process called adhesion (attachment), in
which complementary chemicals such as membrane glycoproteins bind together. Pathogens
3. Ingestion. After phagocytes adhere to pathogens, they extend pseudopods to surround the
4. Phagosome Maturation and Microbial Killing. Lysosomes are membranous organelles
containing digestive chemicals. Killing occurs when lysosomes within the phagocyte fuse
5. Elimination. Phagocytes eliminate remains of microorganisms via exocytosis.
Receptors on the phagocyte’s cell membrane specific for various microbial surface components
Nonphagocytic Killing
Eosinophils, natural killer lymphocytes, and neutrophils can accomplish killing without phago-
cytosis. Eosinophils attack parasitic helminths by attaching to their surface and secreting toxins
and can eject mitochondrial DNA and protein toxins with antimicrobial activity against bacteria.
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
Nonspecific Chemical Defenses Against Pathogens
Chemicals assist phagocytic cells either by directly attacking pathogens or by enhancing other
features of innate immunity. In addition to lysozyme (discussed earlier), they include Toll-like
receptors, NOD proteins, interferons, and complement. Toll-like receptors (TLRs) are integral
proteins of the cytoplasmic membrane of phagocytic cells that recognize molecules shared by
Interferons (IFNs) are protein molecules released by host cells to nonspecifically inhibit the
spread of viral infections. Type I interferons (alpha and beta) are produced in response to TLR
binding of viral nucleic acid. They also cause the malaise, muscle aches, and fever typical of vi-
ral infections. Virally infected monocytes, macrophages, and some lymphocytes secrete alpha
Complement Cascade
Complement factors interact in an amplifying series of chemical reactions in which the products
of one step catalyze the next step. Products of these steps have various functions including act-
Chapter 15 Innate Immunity
Inflammation
Inflammation is a general, nonspecific response to tissue damage resulting from a variety of
causes, including infection with pathogens. Acute inflammation develops quickly and is short
Increased blood flow delivers monocytes and neutrophils to a site of infection. As they arrive,
these leukocytes roll along the inside walls of blood vessels until they adhere to the receptors
lining the vessels, in a process called margination. They then squeeze between the cells of the
Fever
Fever is a body temperature above 37°C. It results when pyrogens, chemicals such as bacterial
toxins, the cytoplasmic contents of bacteria, and antigen-antibody complexes, trigger the hypo-
thalamic “thermostat” to reset at a higher temperature. Body temperature is increased by repeti-