The Immune System and Immunologic Complications

Our immune system is designed to protect us from harmful pathogens; here's what you need to know about how it works.

This CE activity is approved by EMS World Magazine, an organization accredited by the Continuing Education Coordinating Board for Emergency Medical Services (CECBEMS), for 1.5 CEUs. There are two ways to take the CE test that accompanies this article and receive 1.5 hours of CE credit accredited by CECBEMS: 1. Click here to download a PDF of the test. The PDF has instructions for completing the test. 2. Or go to to take the test and immediately receive your CE credit. Questions? E-mail


Review the innate immune system
Describe types of allergic reactions
Discuss conditions associated with immunocompromised patients
Review prehospital care options for these patients

Marie, a paramedic, and her EMT partner, Don, are dispatched to an “unknown medical problem” at a residential address. Upon arrival, they are met at the door by the patient’s wife, who tells them, “My husband doesn’t want to go to the doctor, but I know he’s not feeling well.”

The patient, a 60-year-old male, is lying on the couch complaining of abdominal pain. He says the pain started about four days ago, and describes it as “crampy,” non-radiating, non-reproduceable and 3 on a scale of 0–10. The patient has no other complaints. He has a past medical history of hypertension, lupus and diverticulosis, for which he is prescribed Adalat (nifedipine), prednisone and Imuran (azathioprine). A physical exam reveals slight pain with palpation to the patient’s upper left quadrant and a temperature of 99.2°F. The patient’s vital signs are stable and within normal limits. As Marie attempts to administer oxygen via a nasal cannula, the patient says, “Thanks for coming out here, but I just have a stomach bug and certainly do not need to go to the hospital.”

This situation is certainly plausible, if not common in EMS. The crew is presented with a patient who has a seemingly minor complaint, is stable and does not want to go to the hospital for an evaluation. Understanding both his past medical history and his medications would alert EMS providers that this patient is high risk for infection as a result of an immune system impaired by disease and medication.

Overview of the Immune System

Over millions of years of evolution, the human immune system has evolved into a highly efficient body system designed to protect humans from harmful pathogens (see Figure 1). The immunologic response in humans is coordinated by two separate immune systems: the ancient innate (natural) immune system and the more recently evolved acquired (adaptive) immune system.

The Innate Immune System

The innate immune system is comprised of cells and processes that defend the human host from infection in a nonspecific manner that does not result in long-lasting immunity. Because it does not identify a specific foreign pathogen, innate immunity is sometimes referred to as nonspecific immunity. Innate immunity is thought to be an evolutionarily older defense mechanism (compared to acquired immunity) and is found not only in humans, but is the dominant immune system in other organisms such as plants, insects and fungi. The major functions of the innate immune system include:

  • Local inflammation and recruitment of immune cells to the site of an infection via the production of specialized chemical mediators called cytokines.
  • Activation of the complement cascade, which is composed of plasma proteins that are synthesized in the liver by hepatocytes. These plasma proteins trigger the recruitment of inflammatory cells, identify and “tag” pathogens for elimination, disrupt the plasma membrane of infected cells, thereby killing the pathogen, and help eliminate antigen-antibody complexes.
  • Identification of pathogens in human tissues, organs, blood and lymph by specialized white blood cells such as neutrophils, eosinophils, basophils, mast cells and natural killer cells.
  • Activation of the acquired immune system via the process of antigen presentation.
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