Content Reviewers:Lisa Miklush, PhD, RNC, CNS, Antonia Syrnioti, MD, Viviana Popa, Gabrielle Proper, RN, BScN, MN
Immunity is the ability of the body to fight pathogens, like viruses, bacteria, and fungi; but, also, foreign substances, like toxins and chemicals. Now, the immune system consists of two main branches: innate immunity and adaptive immunity. Innate immunity is the first line of immunity, that we are born with; it is fast, meaning that it responds within several minutes to hours; it’s non-specific, therefore it does not differentiate one pathogen from another; and finally, it’s short-lived, meaning it does not retain the memory of previous infections. On the flip side, adaptive immunity is the second line of defense that is acquired throughout life; it is slower and takes time to respond; but it’s also specific, so it recognizes different pathogens; and long-term, so it doesn’t forget a previous exposure to a pathogen.
Now, let's cover the physiology of the innate immune system, which can be further subdivided into anatomic, physiologic, and cellular components. Anatomic components include physical barriers, like skin and mucous membranes, while physiologic components include additional mechanisms like the microflora found in different parts of the body, like the skin, gut and reproductive organs; or the low stomach pH.
First up, there’s phagocytes, like neutrophils, macrophages and dendritic cells, and these are immune cells that digest pathogens in a process called phagocytosis. There are some key differences between them.
First up, neutrophils are circulating immune cells, so they are normally found in the blood. In fact, they’re the most numerous type of white blood cell in the blood! Microscopically, they also have a multi-lobed nucleus, which is why they’re also called polymorphonuclear neutrophils, or PMN for short. They’re also granulocytes, meaning they have vesicles that store enzymes or proinflammatory compounds. Now, since neutrophils are floating around in the blood, they’re the first immune cells to reach the site of an injury or infection in response to chemical signals, called cytokines, that are produced during acute inflammation.
Now, once they reach the site of injury, neutrophils swallow the pathogen in a process called phagocytosis.
Inside the neutrophil, the pathogen is trapped in a phagosome, which combines with lysosomes which are vesicles that contain antimicrobial enzymes. This creates a phagolysosome that breaks down the pathogen. Some of these enzymes are responsible for the “respiratory burst” where reactive oxygen species like superoxide and hydrogen peroxide are produced. These are highly effective at destroying pathogens, but also damage the neutrophil, causing it to die in the process. Since neutrophils utilize respiratory burst more than other immune cells, they have a very short lifespan, of just a few days.
Neutrophils can also degranulate, meaning they release their enzymes outside the cell to damage extracellular pathogens. However, this also damages themselves and nearby tissue. In addition, they can also release their own proinflammatory molecules like cytokines to attract more immune cells to the area.
Macrophages, on the other hand, are mostly stationary immune cells, so they stay and guard various tissues in the body; they actually derive from circulating white blood cells called monocytes, which travel through the blood and eventually set up shop in one tissue or another. Both monocytes and macrophages are agranulocytes, which means that their cytoplasm doesn’t contain granules. They also have different names depending on their home. So for example, in the lung they’re called alveolar macrophages, while in the liver they’re called Kupffer cells.