Phagocytosis: Study with Video Lessons, Practice Problems & Examples

Phagocytosis, or "cell eating," is a crucial immune response involving six 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, engulf them into a phagosome, and fuse it with lysosomes to form a phagolysosome, where microbes are degraded. Finally, debris is expelled through exocytosis, ensuring the elimination of harmful invaders and maintaining homeostasis in the immune system.

Phagocytosis

This video we're going to begin our lesson on phagocytosis. First, we need to recall from some of our previous lesson videos that phagocytosis is the process of cell eating. In this process, cells are going to be engulfing and digesting material that they bring in from the environment. This includes invading microbes. It turns out that there are many different types of immune system cells that are capable of phagocytosis. This includes macrophages, dendritic cells, and neutrophils, all of which are capable of performing phagocytosis. 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. 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. 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 phagocytosis and inflammation. Now, in this video, we're starting to transition into this part of the map, the innate effector actions, which are going to eliminate invaders, serving as soldiers that are going to be eliminating invaders. 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. Then, after we finish talking about phagocytosis and the details of phagocytosis, 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'll move on to talk more details about phagocytosis. 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. 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.

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.

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. In chemotaxis, phagocytes are recruited to the infection site by chemoattractants like cytokines. During recognition and attachment, phagocytes bind to pathogens using receptors. Engulfment follows, where the phagocyte engulfs the pathogen into a phagosome. The phagosome then fuses with lysosomes to form a phagolysosome, where digestive enzymes degrade the pathogen. Finally, in exocytosis, the debris is expelled from the cell, completing the process.

Which cells are capable of performing phagocytosis?

Several types of immune cells are capable of performing phagocytosis, including macrophages, dendritic cells, and neutrophils. These cells play a crucial role in the immune response by engulfing and digesting invading microbes and other harmful particles. Macrophages are found in tissues and are long-lived, while neutrophils are abundant in the bloodstream and are among the first responders to infection sites. Dendritic cells also perform phagocytosis and are essential for antigen presentation to T cells, bridging innate and adaptive immunity.

What role do chemoattractants play in phagocytosis?

Chemoattractants are crucial in the first step of phagocytosis, known as chemotaxis. They are chemical signals that attract phagocytes to the site of infection. Examples of chemoattractants include cytokines like chemokines and complement component C5a. These molecules can be produced by the invading microbes or by host cells that detect the presence of pathogens. By guiding phagocytes to the infection site, chemoattractants ensure that the immune response is targeted and efficient, facilitating the rapid clearance of harmful invaders.

How do phagocytes recognize and attach to pathogens?

Phagocytes recognize and attach to pathogens through a process involving specific receptors. They can bind directly to pathogen-associated molecular patterns (PAMPs) on the surface of microbes using receptors like mannose-binding lectins (MBLs). Alternatively, they can bind indirectly via opsonins, which are molecules that coat the pathogen and enhance phagocytosis. Opsonins such as C3b from the complement system bind to the pathogen, and phagocytes recognize these opsonins through their receptors, facilitating attachment and subsequent engulfment of the pathogen.

What happens during the formation of the phagolysosome?

During phagolysosome formation, the phagosome containing the engulfed pathogen fuses with lysosomes, which are organelles containing digestive enzymes. This fusion creates the phagolysosome. Toll-like receptors (TLRs) embedded in the phagosome membrane detect the contents of the phagosome before fusion. Once the phagolysosome is formed, the digestive enzymes and reactive oxygen species (ROS) within the lysosomes degrade and destroy the pathogen. This step is crucial for breaking down the pathogen into smaller, non-harmful components that can be expelled from the cell.