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Running head: FOREIGN INVADERS
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Foreign Invaders: The Human Body and Infection
Jessica McClusky
Ferris State University
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Abstract
Humans are exposed to countless microorganisms per day, some pathogenic, and some not. It is
the responsibility of the body to initiate the immune response to fight off any foreign invaders.
An infection results when one of these microorganisms is unable to be eliminated, and begins to
replicate within the body. Infections can be caused by a number of microorganisms, including
bacteria, viruses, fungus, and parasites. The way these are diagnosed and treated varies based on
the pathogen, but hand hygiene still remains a top method for preventing the spread of infections.
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Foreign Invaders: The Human Body and Infection
Humans are exposed to countless numbers of microorganisms every day, some
pathogenic and some not (Giddens, 2012). It is the responsibility of the body’s immune system
to respond and fight off the harmful organisms; if this process is not successful, the pathogen
begins to replicate in the body and an infection occurs. Microorganisms that cause infectious
diseases are the number one cause of death for humans worldwide, especially in developing
countries or areas with poor sanitation (McCance & Huether, 2014). The purpose of this paper is
to describe the assessment details, pathophysiology, clinical manifestations, and
pharmacotherapeutics related to infection and the human body response.
Assessment
The first important step to recognizing or treating a patient with an infection is to take a
thorough health history (Giddens, 2012). The health history will aid the nurse in determining
whether a pathogen should be suspected, based on the individual’s risk factors, history of
infections, and current symptoms or complaints. Risk factors for infection include patients who
are immunocompromised, have chronic diseases, or live in poor environmental conditions
(Giddens, 2012). An immunodeficiency prevents the body from effectively fighting off disease,
while poor living conditions result in a multitude of pathogen exposures for the immune system,
which may overwhelm it. Chronic disease is a risk factor for infection as physiologic changes
caused by disease processes may alter the function of the body’s various defense systems.
Additionally, the numerous medical interventions necessary to manage a chronic illness such as
frequent antibiotics, invasive lines, or surgeries leave the body more susceptible to pathogens.
Ball, Dains, Flynn, Solomon, and Stewart (2015) claims the most important point of the
health history is to determine the issues the patient is experiencing. However, the contributing
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factors may be subtle, making the nurse’s questioning vital in obtaining the whole patient
picture. An in depth infection history would need to include questions related to recent travel, for
example, did the patient recently visit an area with an outbreak of a specific disease or a certain
endemic disease (Giddens, 2012)? Additionally, the patient’s immunization status needs to be
discussed. Knowing which vaccines have been received would help limit the number of diseases
placed on the differential diagnosis list. Finally, recent antibiotic use is important, as antibiotics
can cause a resurgence of certain bacteria. For example, Clostridium difficile tends to show up
after an antibiotic is used that kills the normal flora of the gastrointestinal tract (McCance &
Huether, 2014). This will be discussed more in later sections of this paper.
After the written health history, the nurse is able to complete a physical assessment.
Assessing for infection is difficult, as the assessment findings will vary depending on the
infectious pathogen that is present (McCance & Huether, 2014). Generally, infections begin with
nonspecific symptoms such as malaise, weakness, chills, redness, swelling, fever, and aching
(Giddens, 2012). However, depending on the type of disease suspected from the written health
history, different symptoms may be present. An example would be a gastrointestinal parasite. In
this case, the nurse should be alert for diarrhea, abdominal pain, or nausea for example.
Laboratory tests are a very helpful part of the nurse’s assessment of a patient (Giddens,
2012). With an infection, the nurse should expect to see an elevated white blood cell count, Creactive protein, and erythrocyte sedimentation rate. Additionally, body fluid cultures that are
allowed to grow will identify the pathogen and help direct treatment. Checking the blood for the
presence of antibodies against a certain disease, like Epstein-Barr virus, is an indication that the
disease is or has been present in the body.
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Pathophysiology
Infections in the body can be caused by bacteria, viruses, fungi, or parasites (Giddens,
2012). Within these categories, different species will infect the body in different ways. This
paper will describe the process of infection in as general terms as possible to cover the majority
of the categories. It is important to note that not all microorganisms are pathogenic and will
cause disease (McCance & Huether, 2012). The human gut, for example, is colonized with many
different types of bacteria. These bacteria, called normal flora, digest nutrients from the foods
eaten, and in return provide enzymes that help the body break down complex molecules in to a
stage that can be utilized.
Again highlighting the diverse characteristics of microorganisms is the way in which they
infect their host (McCance & Huether, 2014). For example, the human immunodeficiency virus
(HIV) infects a human body by inserting its genes into healthy host cells. This causes the cells to
become defective and lose function, and also to pass the genetic flaw on when it replicates. On
the other hand, Clostridium difficile causes the person to be sick through release of a toxin. The
toxin is actually harmful, not the actual bacteria.
In order to cause an infection, a pathogen must complete four stages. These stages are
colonization, invasion, multiplication, and spread (McCance & Huether, 2014). Colonization
involves the introduction of the pathogen to the body. This can be achieved through transmission
from one source to another (Giddens, 2012). Transmission occurs when a microorganism moves
from a portal of exit in the reservoir it is currently living in, to a portal of entry in a new potential
host. A portal of exit can be urine, feces, saliva, blood, skin, or the gastrointestinal tract. Portals
of entry to the new host include broken skin, sexual contact, mouth, gastrointestinal tract, or
contaminated food or water.
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While colonization is the presence of the pathogen, invasion occurs when the
microorganism is able to resist the immune response of the host, and attaches to host cells
(McCance & Huether, 2014). All the cells in the human body have antigens that designate it as
innate. Foreign pathogens, though, do not have these same markers, and are recognized as
invaders. This activates the inflammatory and immune response, which are the lines of defense to
protect the body (McCance & Huether, 2014). The immune response is a very complex process,
but its important role in the success or failure of a pathogen to infect makes it important to
discuss. Macrophages are one of the first responders to a foreign invader in the body. They
engulf the invaders through phagocytosis. Helper T cells are activated by the macrophages, and
in turn activate B cells. These B cells produce antibodies, which either kill the microorganisms,
or make phagocytosis more effective, while killer T cells also work to destroy the pathogen.
Finally, B and T cells can become memory cells, which are able to remember and recognize
antigens on pathogens the body has been exposed to more quickly, and consequently can create a
quicker response on subsequent exposures. With an infection, invasion has occurred when a
pathogen is able to resist these host defenses.
Next, in order to be considered an infection, the pathogen must multiply within the body
in order to reach numbers that will cause significant destruction to the human body (McCance &
Huether, 2014). The ability to replicate is accomplished by either using the nutrients provided by
the host’s body, or by utilizing host cell structures. An example of this would be HIV, which
inserts genes into host cells, and relies on the replication of those host cells to pass on the
defective genes.
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The final stage of infection is spread (McCance & Huether, 2014). This is when the body
begins to show clinical signs of the disease. This can be a local, such as a skin, infection or
systemic like in the case of a bacteremia.
Pathogens, however, also have their own defense mechanisms used to fight back against
the immune system in the body (McCance and Huether, 2014). Some, like staphylococcus or
streptococcus produce toxins that kill phagocytes. Another mechanism is antigenic variation,
where pathogens are able to alter their surface antigens. This results in a less specific, and
therefore less effective, immune system response. Influenza is an example of the mechanism of
antigenic drift, where the antigens change each year, preventing the body from developing the
correct memory cells to quickly and efficiently combat the next year’s strain.
Clinical Manifestations
The clinical manifestations seen with infectious diseases are dependent on the type of
pathogen and the area of the body that is infected (McCance & Huether, 2014). For the purposes
of this section, general infection manifestations will be discussed. However, it is important to
note that certain pathogens require a unique set of symptom discussions. For example, when
patient history raises concern about a Clostridium difficile infection, manifestations would
include watery diarrhea, nausea, and abdominal pain (CDC, 2013).
Fever is one of the most recognized and hallmark signs of infection (McCance &
Huether, 2014). Body temperature is controlled in the hypothalamus in the brain. When the body
is working to fight an infection and is utilizing the immune response, cytokines are released.
These cytokines, including interferon and tumor necrosis factor, are similar to hormones, and
have an effect on the thermoregulatory tissues in the body. However, the body is not failing to
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regulate its temperature correctly but rather regulating it at a high set point based on these
cytokines being released.
Laboratory abnormalities are found with infection processes in the body as well. The
activation of the immune and inflammatory responses generally cause an increase in the white
blood cell count in the patient’s blood (McCance & Huether, 2014). Since the inflammatory
response signals the production of more neutrophils, the patient’s blood will show a greater
proportion of immature neutrophils such as band cells and metamyelocytes, than those in the
mature state. Additional laboratory tests would include a positive result on a culture and
sensitivity, where samples taken from the area of suspected infection grow the offending
pathogen under laboratory conditions (Giddens, 2012). The grown bacteria are then tested to
determine which antibiotics they are resistant or sensitive to, helping the providers to determine
the most effective medication.
Redness and/or heat are additional signs of infection, especially a localized infection
(McCance & Huether, 2014). Once cell injury from a pathogen begins to occur, the
inflammatory response is triggered. After the activation of the plasma systems, the complement
and clotting kinin pathways begin and microscopic changes to the vascular system results. One
of these changes is vasodilation, which allows for greater blood flow, thus causing the redness
and heat generally seen.
When an infection goes untreated, or the host’s immune system is unable to fight it, the pathogen
can cause significant damage to the body (Giddens, 2012). Without proper treatment or
supportive care, the body can succumb to death. However, there are compensatory mechanisms
in place to assist the body while waiting for the immune response to be successful (Giddens,
2012). Vascular permeability increases, which causes hypotension and hypovolemia. The
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nervous system responds with vasoconstriction and shunting blood to solely essential
organs. The heart rate will increase to try to maintain cardiac output. The kidneys filter
fluid back to the body to counteract the hypovolemia, resulting in decreased or no urine
output. The respiratory system triggers an increase in respiration rate in an attempt to
oxygenate more tissues. Eventually, along with cardiac decompensation, inadequate
perfusion results, leading to changes in mental status like confusion or seizures. If the
infection is still progressing, the body will move to multisystem organ failure and possibly
death.
Pharmacology
Before discussing the pharmacology principles that are used to treat infections, it is
important to mention the impact that hand washing has on the spread of infection. Larson (1988)
established a causal link between hand washing and a decrease in infection transmission. The
Centers for Disease Control and Prevention (CDC) also considers hand hygiene the top
intervention for preventing the transmission of microorganisms in the healthcare setting (CDC,
2014).
Another important aspect of prevention of infection is the use of vaccinations.
Worldwide, vaccines have eradicated smallpox completely (Lehne, 2013). In the United States,
pertussis, mumps, and tetanus have been reduced drastically, while diphtheria, polio, rubella, and
measles have been almost eliminated. Vaccinations can be comprised of a killed microorganism,
live and attenuated organism, or toxoid depending on which disease it is developed for. The
general mechanism of action remains the same regardless of the type of immunization. When
introduced to the body, these non-infectious forms of pathogens stimulate an immune response.
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Antibodies are formed, which are prepared to fight if the body ever encounters the actual
pathogenic organism.
If the human body is not able to fight off an invading organism, antimicrobials are the
necessary therapy. Choosing an antimicrobial, and also understanding its mechanism of action, is
dependent on the type of pathogen that has been identified as causing disease (Lehne, 2013). For
example, penicillins target the cell wall of bacteria by either inhibiting cell wall synthesis, or
promoting disruption of the cell wall. This causes a weaker cell, making it more susceptible to
lysis and death. Other antibiotics, like amphotericin B, increase cell membrane permeability.
This causes the contents of the cell to leak out, leading to cell death. For a viral infection,
antivirals will work to suppress replication, which is vital as the pathogen is unable to live
without a host cell. Specific enzymes, like DNA polymerase, are inhibited with these drugs,
causing viral replication to come to a halt.
When categorizing antibiotics, there are both narrow-spectrum and broad-spectrum
antibiotics (Lehne, 2013). Broad-spectrum antibiotics are effective against both gram-positive
and gram-negative organisms, while narrow-spectrum are much more specific in the
microorganisms they target. While determination of the invading organism is a priority in
treating infection, it is not always possible for it to occur immediately. In this case, a broadspectrum antibiotic would be utilized, until the pathogen can be identified, and the appropriate
narrow-spectrum drug chosen. The reason for this is the emergence of antibiotic resistant
organisms. These organisms become resistant by developing a mechanism to block the
effectiveness of the antibiotic (McCance & Huether, 2014). For example, Penicillin resistance
emerges when the bacteria produce and enzyme called beta lactamase, which breaks down the
structure of the antibiotic, making it ineffective. The rise in antibiotic resistance can be related to
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overuse of antibiotics, such as for viral illnesses, that cause destruction of the good normal flora
in the body (Aziz, 2013). Also, it is common for patients to neglect to complete a full prescribed
course of antibiotics. This allows for the resurgence of the bacteria that may have had a slight
resistance to the drug, giving them the opportunity to evolve in to fully resistant organisms
(McCance & Huether, 2014).
In conclusion, infection remains a common disease encountered by those in the
community and hospital settings. Though a complex process specific to the type of invading
organism, the understanding of and appropriate antibiotic or antimicrobial use is vital in
effectively treating and overcoming the disease. Still, simple interventions such as hand washing
have been shown to be a necessary and successful first line defense.
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References
Aziz, A. (2013). The role of healthcare strategies in controlling antibiotic resistance. British
Journal Of Nursing, 22(18), 1066-1074.
Ball, J. W., Dains, J. E., Flynn, J. A., Solomon, B. S., & Stewart, R. W. (2015) Seidel’s guide to
physical examination (8th ed.). St. Louise: Elsevier.
Centers for Disease Control and Prevention. (2013). Clostridium difficile infection. Retrieved
from http://www.cdc.gov/HAI/organisms/cdiff/Cdiff_infect.html
Centers for Disease Control and Prevention. (2014). Hand hygiene in healthcare settings.
Retrieved from http://www.cdc.gov/handhygiene/
Giddens, J. F. (2012). Concepts for Nursing Practice. St. Louis: Elsevier.
Larson, E. (1988). A causal link between handwashing and infection? Examination and evidence.
Infection Control and Hospital Epidemiology, 9(1).
Lehne, R. A. (2013). Pharmacology for nursing care (8th ed.). St. Louis: Elsevier.
McCance, K. L. & Huether, S. E. (2014). Pathophysiology (7th ed.). St. Louis: Elsevier.