Host Defenses
Physical Factors
Skin
• Epidermis consists of tightly packed cells with
• Keratin, a protective protein
• Sweat
• Sebum
Mucous membranes
• mucus
Physical Factors
• Ciliary escalator:
Microbes trapped in mucus
are transported away from
the lungs.
• Lacrimal apparatus:
Washes eye.
• Saliva: Washes microbes off.
• Urine: Flows out.
• Vaginal secretions: Flow out.
Chemical Factors
• Fungistatic fatty acid in sebum.
• Low pH (3-5) of skin.
• Lysozyme in perspiration, tears, saliva, and tissue fluids.
• Low pH (1.2-3.0) of gastric juice.
• Transferrins in blood find iron.
• NO inhibits ATP production.
• Mucus
Normal Microbiota
• Microbial antagonism/competitive exclusion: Normal microbiota
compete with pathogens.
Types of Pathogens
• Bacteria
• Major entry points through direct bodily
contact, open wounds, inhalation, and
ingestion (c, d, e, f, and g)
• Viruses
• Nucleic acids enclosed in a protein coat and
some enclosed by an membrane envelope (a
and b)
• Must infect host cell to replicate
• May kill host cell rapidly or lie dormant
• May cause cancer
• Eukaryotic parasites
• Protists (k), fungi (h, i, and j), worms (l)
• Damage host by using host nutrients or
secreting toxic chemicals
Innate and Adaptive Defenses
Components of Whole Blood
Formed Elements in Blood
Formed Elements in Blood
Differential White Cell Count
• Percentage of each type of white cell in a sample of 100
white blood cells.
Neutrophils
60-70%
Basophils
0.5-1%
Eosinophils
2-4%
Monocytes
3-8%
Lymphocytes
20-25%
White Blood Cells
• Neutrophils: Phagocytic
• Basophils: Produce histamine
• Eosinophils: Toxic to parasites and some phagocytosis
• Dendritic cells: Initiate adaptive immune response
• Monocytes: Phagocytic as mature macrophages
• Fixed macrophages in lungs, liver, and bronchi
• Wandering macrophages roam tissues.
• Lymphocytes: Involved in specific immunity.
Phagocytosis
• Phago: from Greek,
meaning eat
• Cyte: from Greek, meaning
cell
• Ingestion of microbes
or particles by a cell,
performed by phagocytes.
Phagocytosis
Toll-like Receptors (TLRs)
• Macrophages and cells lining the gastrointestinal and respiratory
tracts bear TLRs
• TLRs recognize specific classes of infecting microbes
• Activated TLRs trigger the release of cytokines that promote
inflammation
The Complement System
• Serum proteins
activated in a
cascade.
Effects of Complement Activation
• Opsonization or
immune adherence:
Enhanced
phagocytosis.
• Membrane attack
complex: Cytolysis.
• Attract phagocytes.
Effects of Complement Activation
Classical Pathway
Alternative Pathway
Lectin Pathway
Some Bacteria Evade Complement
• Capsules prevent C activation.
• Surface lipid-carbohydrates prevent MAC formation.
• Enzymatic digestion of C5a.
Interferons (IFNs)
Interferons (IFNs)
• Alpha IFN and Beta IFN: Cause cells to produce antiviral
proteins that inhibit viral replication.
• Gamma IFN: Causes neutrophils and macrophages
to phagocytize bacteria.
Microbial Evasion of Phagocytosis
Inhibit adherence: M protein,
capsules
Streptococcus pyogenes, S. pneumoniae
Kill phagocytes: Leukocidins
Staphylococcus aureus
Lyse phagocytes: Membrane
attack complex
Listeriamonocytogenes
Escape phagosome
Shigella
Prevent phagosome-lysosome HIV
fusion
Survive in phagolysosome
Coxiella burnetti
Inflammation
• Redness
• Pain
• Heat
• Swelling (edema)
• Acute-phase proteins activated (complement, cytokine, and kinins)
• Vasodilation (histamine, kinins, prostaglandins, and leukotrienes)
• Margination and emigration of WBCs
• Tissue repair
Chemicals Released by Damaged Cells
Histamine
Vasodilation, increased permeability
of blood vessels
Kinins
Vasodilation, increased permeability
of blood vessels
Prostaglandins
Intensity histamine and kinin effect
Leukotrienes
Increased permeability of blood vessels,
phagocytic attachment
Inflammation
Inflammation
Fever: Abnormally High Body Temperature
• Hypothalamus normally set at 37°C.
• Gram-negative endotoxin cause phagocytes to release
interleukin–1 (IL–1).
• Hypothalamus releases prostaglandins that reset the
hypothalamus to a high temperature.
• Body increases rate of metabolism and shivering which
raise temperature.
• When IL–1 is eliminated, body temperature falls (crisis).
Fever
• Advantages
• Increase transferrins
• Increase IL–1 activity
• Disadvantages
• Tachycardia
• Acidosis
• Dehydration
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