Phagoctytosis: Study with Video Lessons, Practice Problems & Examples

Phagocytosis is a vital immune process involving six key steps: chemotaxis, recognition and attachment, engulfment, phagolysosome formation, destruction and digestion, and exocytosis. Phagocytes, such as macrophages and neutrophils, are recruited to infection sites by chemoattractants like cytokines. They recognize and bind to pathogens using mannose-binding lectins or opsonins, engulfing them to form a phagosome. This fuses with lysosomes to create a phagolysosome, where pathogens are degraded by hydrolytic enzymes and reactive oxygen species. Finally, debris is expelled through exocytosis, aiding in pathogen elimination.

Phagoctytosis

This video we're going to begin our lesson on phagocytosis. And so first, we need to recall from some of our previous lesson videos that phagocytosis is the process of cell eating. And so really in this process cells are going to be engulfing and digesting material that they bring in from the environment, and this includes invading microbes. So it turns out that there are many different types of immune system cells that are capable of phagocytosis. And these include macrophages, dendritic cells, and neutrophils, all of which are capable of performing phagocytosis. And so if we take a look at this image that we have down below, notice that it is our map of the lesson on innate immunity. And already in our previous lesson videos, we've talked about the first line defenses, several of the second line defenses, the cells of immunity, and we've talked about the scanning systems including cell communication, pattern recognition receptors, and the complement system. And we also mentioned how the complement system can actually serve as a scanning system to detect signs of microbes, but upon activation it can actually trigger several innate effector actions, such as for example phagocytosis and inflammation. And so now in this video we're starting to transition into this part of the map, the innate effector actions, which recall are going to eliminate invaders, and so they serve as soldiers that are going to be eliminating invaders. And so the innate effector actions that we're going to talk about moving forward are going to be phagocytosis, which is again the main focus of this video. And then after we finish talking about phagocytosis and the details of phagocytosis, then we'll move on to talk more about the details of inflammation, the details of fever, and the details of the interferon response. But for now we will move on to talk more details about phagocytosis. And so this here concludes our brief introduction to phagocytosis, and we'll get to learn more about the steps of phagocytosis in our very next lesson video. So, I'll see you all there.

Steps of Phagocytosis

In this video, we're going to discuss the steps of phagocytosis. And so the process of phagocytosis involves a series of 6 steps that we have numbered down below 1 through 6. And each of the steps that you see in the text corresponds with the image that we have down below as well. And so in the very first step of phagocytosis is chemotaxis, which recall from some of our previous lesson videos is a process that involves the movement of a cell either towards or away from a chemical. In this case, it's going to be towards a chemical. And so in this very first step of phagocytosis and the process of chemotaxis, phagocytes are going to be recruited. So phagocytes are going to be recruited to the site of infection by chemoattractants. For example, cytokines like chemokines or C5a is an example of a specific chemoattractant. And so this will allow for phagocytes to move to the site of infection. And so if we take a look at our image down below, notice that it's an image focused on the steps of phagocytosis. And notice that in the very first step of phagocytosis we have chemotaxis. So we can go ahead and label, this yellow circle as number 1 because this is the very first step of phagocytosis, chemotaxis. And so chemoattractants are going to be produced, like for example, C5a here serves as a chemoattractant. And the chemoattractant can be produced either by the microbes themselves as products released by the microbes, or the chemoattractants could be released by our own host cells that detect signs of the invading microbes. But either way, the chemoattractants are going to be released, and that is going to help to attract the phagocyte to the location.

Now in the second step of phagocytosis is recognition and attachment. And as its name implies, this is going to be when the phagocyte needs to recognize the invader and bind to the invader. And so phagocytes are going to bind to the microbe, the invading microbe, and it can do this directly via MBLs, mannose-binding lectins, which will bind to mannose carbohydrates on the surface of microbes, or it could bind to the microbes indirectly via opsonins, which recall from our previous lesson videos that opsonins are molecules that will bind to the surface of microbes and allow for opsonization, improved phagocytosis. And so if we take a look at our image down below, notice that the second step of phagocytosis, is going to be recognition and attachment. And so notice that this could involve opsonins like for example C3b can serve as an opsonin to coat the surface of the microbe and allow for improved phagocyte binding and efficiency.

And so after recognition and attachment, the third step of phagocytosis is engulfment. And as its name implies, this is going to be when the phagocyte is going to engulf the microbe. So the phagocyte will send out pseudopods, which are these long projections, to surround and engulf the material, creating what we refer to as a phagosome. And so the phagosome is, basically just going to be a membrane structure that results from phagocytosis that is going to contain the microbe that was phagocytosed. And so if we take a look at our image down below, notice that the third step of phagocytosis is going to be engulfment. And so we can label it with number 3. And so notice that the phagocyte, which is, here in blue, this big macrophage right here, is going to be able to bind to the microbe using those opsonins that are coating the microbe, and it will use its C3b receptors to, again, bind to the microbe and get rid of the microbe, and, ingest the microbe, I should say. And so when this microbe is engulfed, when it is ingested, it is going to be surrounded by a membrane, and that membrane we refer to as the phagosome.

So after engulfment, after the microbe has been engulfed, the 4th step of phagocytosis is going to be phagolysosome formation. And so what's going to happen is that Toll-like receptors or TLRs that are embedded in the phagosome are going to be able to detect the phagosome's contents. And, it will be able to do that before the phagosome fuses with lysosomes. And recall from our previous lesson videos that lysosomes are cell structures that contain digestive enzymes that are capable of, destroying and degrading substances. And so if we take a look at our image down below, what we'll see is that the phagosome that results from phagocytosis and contains the microbe is going to fuse with lysosomes in the 4th step. And so the 4th step is going to be phagolysosome formation. And so the phagolysosome is going to be the fusion of lysosomes, which notice lysosomes are these, little structures that you can see, right here in the image. And these lysosomes will fuse with the phagosome to form the phagolysosome. And so those digestive enzymes that are contained within the lysosomes will fuse with the phagosome, and they will be able to start to degrade and break down the microbe.

And that leads us to the 5th step of phagocytosis, which is going to be the destruction and digestion of the microbe. And so the lysosome components, including the hydrolytic enzymes, those digestive enzymes, and as well as ROS or reactive oxygen species that are found within the lysosomes, those are going to degrade the invader as the pH within the phagolysosome begins to decrease. And so that all helps to degrade the microbe and destroy the microbe. And so notice down below in the 5th step of phagocytosis, we have the destruction and digestion of the microbe. And so, notice that the microbe is being completely destroyed within the phagolysosome.

And in the 6th step, of phagocytosis, we have is exocytosis. And so what happens is the phagolysosome is going to fuse with the cell's cytoplasmic membrane to release the debris that resulted from the degradation of the microbe. And so notice down below in the 6th and final step down below right here, we have exocytosis. And so the phagolysosome will fuse with the cell cytoplasmic membrane and will release the degraded microbe. And so now the microbe is no longer capable of causing this harm, and we will be able to eliminate it from our bodies after this process. And so, in the case that the invading microbes are actually not cleared immediately, it is also possible for these macrophages or phagocytes, to produce cytokines, and, those cytokines can, continue to recruit more and more phagocytes over time. So more and more phagocytes will be recruited to the location so that all of those phagocytes can work together to eliminate all of the microbes in the infected area. And so this here concludes our brief lesson on the steps of phagocytosis, and something that's important for you to be able to understand is, the order of these steps. That is something that you should be able to do as you prepare for one of your microbiology exams on this process. And so make sure that you're able to put these steps in the correct order. And so this here concludes our brief lesson on the steps of phagocytosis, and we'll be able to apply these concepts as we move forward. So I'll see you all in our next video.

Where are bacteria killed by enzymes and toxic substances once ingested by a white blood cell?

• A. Nucleus.
• B. Phagolysosome.
• C. Lysosome.
• D. Cytoplasm.
• E. Phagosome.

Which of the following statements about phagosomes and phagolysosomes are true?

• A. Phagosomes are vesicles surrounding an engulfed microbe.
• B. Phagosomes can be found in neutrophils, macrophages, and T cells.
• C. Phagolysosomes destroy the engulfed microbe with digestive enzymes.
• D. Phagolysosomes are the fusion of a phagosome and a lysosome.
• E. A and C.
• F. B and D.
• G. A, C, and D.
• H. All of the above.

Which of the following statements about phagocytosis is incorrect?

• A. Digestion of the pathogen occurs in the phagolysosome.
• B. Cells capable of phagocytosis have receptors that recognize C3b proteins or antibodies bound to the pathogen.
• C. Cells capable of phagocytosis move toward the infected area of the body via chemotaxis.
• D. Macrophages die after phagocytosis of a pathogen while neutrophils regenerate and survive.

Some pathogens create C5a peptidase, an enzyme that destroys C5a proteins. Which of the following is not a benefit that the pathogen would experience after destroying C5a proteins?

• A. Without C5a, the pathogen is less likely to attract a phagocytotic cell via chemotaxis.
• B. Without C5a, complement proteins and immune cells are less likely to come to the site of infection.
• C. C5a would not be able to bind to the pathogen and trigger opsonization.
• D. Without C5a, mast cells are less likely to release histamines & cytokines which trigger inflammation.

Here’s what students ask on this topic:

What are the steps involved in phagocytosis?

Phagocytosis involves six key steps: chemotaxis, recognition and attachment, engulfment, phagolysosome formation, destruction and digestion, and exocytosis. During chemotaxis, phagocytes are recruited to the infection site by chemoattractants like cytokines. In recognition and attachment, phagocytes bind to pathogens using mannose-binding lectins or opsonins. Engulfment follows, where the phagocyte engulfs the pathogen to form a phagosome. This phagosome fuses with lysosomes to create a phagolysosome, where hydrolytic enzymes and reactive oxygen species degrade the pathogen. Finally, exocytosis expels the debris, aiding in pathogen elimination.

What role do macrophages play in phagocytosis?

Macrophages are crucial phagocytes in the immune system. They are responsible for engulfing and digesting pathogens and cellular debris. During phagocytosis, macrophages are recruited to infection sites by chemoattractants. They recognize and bind to pathogens using receptors for mannose-binding lectins or opsonins. Once bound, macrophages engulf the pathogen, forming a phagosome. This phagosome fuses with lysosomes to create a phagolysosome, where digestive enzymes and reactive oxygen species degrade the pathogen. Finally, macrophages expel the debris through exocytosis, helping to eliminate the infection.

How do chemoattractants function in phagocytosis?

Chemoattractants are chemical signals that guide phagocytes to infection sites during phagocytosis. They can be produced by invading microbes or host cells that detect microbial presence. Common chemoattractants include cytokines like chemokines and complement component C5a. These molecules create a chemical gradient that phagocytes follow, moving towards higher concentrations of the chemoattractant. This process, known as chemotaxis, ensures that phagocytes reach the site of infection efficiently, where they can then recognize, engulf, and destroy pathogens.

What is the significance of the phagolysosome in phagocytosis?

The phagolysosome is a critical structure in phagocytosis, formed by the fusion of a phagosome with lysosomes. This fusion allows digestive enzymes and reactive oxygen species within the lysosomes to enter the phagosome, creating an environment that degrades and destroys the engulfed pathogen. The acidic pH and enzymatic activity within the phagolysosome ensure efficient breakdown of the pathogen. This process is essential for eliminating microbial invaders and preventing infection. After degradation, the debris is expelled from the cell through exocytosis.

What are opsonins and how do they enhance phagocytosis?

Opsonins are molecules that enhance phagocytosis by marking pathogens for recognition by phagocytes. They bind to the surface of microbes, making them more attractive to phagocytes. Common opsonins include antibodies and complement components like C3b. When opsonins coat a pathogen, they facilitate its recognition and attachment by phagocytes, which have receptors for these opsonins. This process, known as opsonization, significantly improves the efficiency of phagocytosis, ensuring that pathogens are quickly and effectively engulfed and destroyed by phagocytes.