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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.
The cellular components of innate immunity include several types of white blood cells, all of which are produced in the bone marrow from hematopoietic stem cells which give rise to all blood cells.
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.
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