Video Transcript
The figure shows the cellular response to tissue injury after the skin has been broken by a nail. What process is illustrated by cell X?
You may recall that a pathogen is an organism or agent that can infect a host and cause disease. You’re probably familiar with the flu, which is caused by the influenza virus. And if you’ve ever had food poisoning, it may well have been caused by a bacterium called Salmonella. Our bodies are covered with skin, which acts as a physical barrier to prevent pathogens such as these from getting inside. However, if the skin is broken, for example, with a nail, as we can see in the figure, pathogens present on the nail can use this opening to enter the body and establish an infection.
Cells of the nonspecific immune system must then be activated in order to stop the pathogen from spreading throughout the body and causing serious damage. First, mast cells are activated. These are specialized white blood cells which look granular because they contain small particles filled with chemicals that stimulate a process known as inflammation. Inflammation begins when activated mast cells release a chemical called histamine, represented here as yellow dots. They also release cytokines, which are represented here as pink dots.
Histamine causes the blood vessels near the site of infection to dilate and to become more permeable, while cytokines stimulate phagocytes, another type of white blood cell, to travel in the blood to the infected area. Because of the increased blood flow and permeability of the blood vessels, lots of phagocytes are able to migrate to the site of injury really quickly.
Once the phagocytes have located the invading pathogens, they engulf them in a process called phagocytosis. This involves digesting the pathogen inside the phagocyte using enzymes and sometimes even presenting its antigens on the cell surface membrane of the phagocyte to activate components of the specific immune response.
If we now look back at the figure in the question, we can see that cell X has left the blood vessel at the site of infection and is in the process of engulfing a pathogen. We can therefore deduce that the process illustrated by cell X is phagocytosis.
