My understanding of the bodies defense systems can be
broken down into three major lines of defense (the first
two being a part of the innate immune response) as
talked about by McCance and Huether (2006);The first
line of defense is characterized by physical,
mechanical, biochemical barriers to invasion by
pathogens. Physical defenses can be characterized as
those protecting the host from the outside environment
through tightly associated epithelial cells of the skin,
menbraneous linings of the gastrointestinal system, the
genitourinary tract, and the respiratory tract.The
mechanism by which the latter systems are able to defend
themselves include; sloughing of dead
skin,sneezing,vomitting,urination,mucociliary clearance,
and low body temperatures to name a few. As in the text,
and further noted in the module for Unit 2 it is
interesting to note that microorganisms favor 37C for
their survival. Biochemical barriers include mucous,
prespiration, saliva, and tears.It was interesting to
note that sweat,tears,and saliva contain lysozomal
enzymes that can attack the cell wall of Gram +
bacteria, whereas sabaceos glands on the skin secrete
antibacterial and antifungal fatty acids and lactic
acids creating an inhospitable environment on the skin,
with an acidic pH of 3 to 5. A common issue in the
clinical seting that often presents itself is invasion
by pathogens due to a decrease in normal bacterial flora
as caused by the use of antibiotics.  The second line of
defense is a part of the inflammatory response and
phagocytosis. The inflamatory response helping to
protect, promote healing, prevent infection. It is
typically recognized as stated in the Unit 2 module
as;"redness, swelling, heat, pain, and loss of function.
These clinical manifestations result from increased
blood flow (hyperemia), increased vascular permeability,
and the movement of white blood cells to the area of
injury" (p. 1a). The third line of defense is recognized
as specific immune responses as with Natural Killer
Cells. Aquired immunity can be considered another form
of defense, as provided by vaccinations.

Despite our defenses, invaders can be succesfull and
evade our host defenses. Using the presentation of a UTI
I will explain how.The periurethral mucous secreting
glands surrounding the two-thirds of the female urethra
acts to trap the bacteria before it can ascend to the
bladder, in addtion the urethral sphincter should act as
an added mechanical defence. However, at times bacteria
ascends to the bladder and the immune response time may
not be fast enough to stop the reproduction of the
pathogens.As noted some immune responses may take up to
a week to respond, whereas Norwalk virus for example has
a incubation period of 24-48hrs(McCance & Huether,
2006). Furhter pathogen survival can be broken down as
follows; Bacterial: Surface coats-inhibiting
phagocytosis; Surface receptors that bind to host cells;
Toxins that damage cells or alter their function such as
exotoxins, which damage cell menbrane, activate second
messengers, inhibit protein synthesis. Antigen variation
can also explain how pathogens change appearances by
altering surface molecule (antigen) thus making it more
difficult for the specificity of the immune system to
recognize invaders. This occurs by mutation,
recombination, and gene switching. (McCance & Huether,
2006).Other examples of clinically relevant evading host
barriers include; Bacteria that produce thick capsule of
carbohydrate or protein that are antiphagocytic,
Tubercle Bacillus surrounded by waxy capsule,
Streptoccocus has a long M wall that supresses
complement activation. Viruses are smarter and bypass
defense mechanisms by developing intracellularly,
thereby hiding within cells avoiding normal or
inflammatory immune responses. Fungi has a  pathological
adaptation to host cell, where it can colonize on skin
and digest keratin, grow in wide temperature
variations,live in lower O2 environments and suppress
immune defenses.

The way in which the immune system initially responds to
invaders is by recognizing invasion. The scavenger cells
such as neutrophils are the first to arrive to attack
bacteria and engulf them. The complement system is then
activated attaching proteins to bacteria surface (C3b
attaches to surface of bacterium with carbohydrate
capsule)facilitating phagocytosis. Macorraphes aid in
pathogen digestion and signal T cells, which activate B
cells to produce antibodies for further destruction of
pathogens. Eventually T cells help to slow/stop immune
response, and B and T cells become become "memory cells"
in the event of a re-invasion(McCance & Huether, 2006,
p.297 Fig 9-2).