In this video, we're going to discuss the steps of phagocytosis. The process of phagocytosis involves a series of 6 steps that we have numbered from 1 through 6. Each of the steps you see in the text corresponds with the image below. In the very first step, chemotaxis, which we recall from some of our previous lesson videos, involves the movement of a cell either towards or away from a chemical. In this case, it's going to be towards a chemical. Phagocytes are going to be recruited to the site of infection by chemoattractants, such as cytokines like chemokines or C5a, which is an example of a specific chemoattractant. This will allow phagocytes to move to the site of infection. The image below focuses on the steps of phagocytosis, labeling the yellow circle as number 1 for chemotaxis. Chemoattractants are produced either by the microbes themselves or by our own host cells that detect signs of the invading microbes. These chemoattractants help attract the phagocyte to the location.
The second step of phagocytosis is recognition and attachment. Phagocytes need to recognize the invader and bind to it. This can occur directly via MBLs (Mannose Binding Lectins), which bind to mannose carbohydrates on the surface of microbes, or indirectly via opsonins. Opsonins, as we discussed in our previous lessons, are molecules that bind to the surface of microbes and improve phagocytosis. An example of an opsonin used is C3b, which coats the surface of the microbe to enhance phagocyte binding and efficiency. The second step, recognition and attachment, is shown in our images.
The third step is engulfment. The phagocyte sends out pseudopods, long projections, to surround and engulf the microbe, creating what we call a phagosome. This is a membrane structure that contains the microbe. We see the blue-colored macrophage in the image using its C3b receptors to bind to the microbe.
The fourth step is phagolysosome formation. Toll-like receptors (TLRs) embedded in the phagosome detect its contents before the phagosome fuses with lysosomes. Lysosomes are cell structures containing digestive enzymes capable of destroying substances. In this step, the phagosome containing the microbe fuses with the lysosomes to form the phagolysosome. This fusion begins the process of degrading the microbe.
The fifth step of phagocytosis is the destruction and digestion of the microbe. The hydrolytic enzymes and reactive oxygen species (ROS) within the lysosomes degrade the invader as the pH within the phagolysosome decreases, helping to destroy the microbe.
In the sixth and final step, exocytosis occurs. The phagolysosome fuses with the cell's cytoplasmic membrane to release the debris from the degraded microbe. This final step ensures that the microbe can no longer cause harm and is eliminated from the body. If invading microbes are not cleared immediately, it's possible for macrophages to produce cytokines, which continue to recruit more phagocytes to the location. This helps to eliminate all microbes in the infected area.
This concludes our lesson on the steps of phagocytosis. Understanding the order of these steps is important as you prepare for a microbiology exam. Make sure you are able to sequence these steps correctly. We will apply these concepts as we move forward. I'll see you all in our next video.
