Phagocytosis

What types of cells perform phagocytosis?

Unicellular organisms perform phagocytosis, but instead of destroying infected cells or viruses, they use it to eat.

Fig. 1 - Diagram of the unicellular amoeba as it consumes its food

Multicellular organisms use phagocytosis as an immune response. The different cells that perform phagocytosis are macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts.

The cells used in multicellular phagocytosis

• Macrophages are white blood cells that use phagocytosis on any cell that does not have proteins specific to the organism it lives in. Some of the cells they destroy are cancer cells, cellular debris (what is leftover from when a cell dies), and foreign substances like pathogens (viruses, bacteria, and toxins that infect an organism). They have also been seen protecting tissues and potentially helping with the formation of brains and hearts in organisms.

• Neutrophils are also white blood cells and make up 1% of the body’s total blood cells. They are created inside the bone marrow and have to be replaced daily due to their short lifespan. They are the first cell to respond to any sort of issue in the immune system like an infection or wound.

• Monocytes are another type of white blood cell made in the bone marrow. They make up 1 to 10% of the body’s white blood cell count. Eventually, they can differentiate into macrophages, osteoclasts, and dendritic cells once they travel from the blood into the tissues. They also play a role in adaptive immunity via inflammatory and anti-inflammatory responses.

• Dendritic cells are called antigen-presenting cells because of their role. After transforming from monocytes, they remain in the tissues and move infected cells to T cells, another white blood cell that destroys pathogens in the body.

• Osteoclasts are cells with multiple nuclei that are formed from the fusion of cells derived from the monocytes found in the bloodstream. Osteoclasts work to destroy and rebuild the bones in the body. The bone is destroyed via secreted enzymes and ions. Osteoclasts perform their phagocytosis by consuming the bone fragments created by the enzymes and ions. Once the bone fragments are consumed, their minerals are released into the bloodstream. Another type of cell, osteoblasts, can help regenerate bone cells.

What are the steps of phagocytosis?

1. Phagocytic cells are on standby until an antigen or a messenger cell that originates from within the organism’s body, such as complement proteins or inflammatory cytokines, is discovered.

2. The phagocytic cell moves towards a high concentration of cells, pathogens, or ‘self cells’ that have been released from being attacked by pathogens. This movement is known as chemotaxis. Occasionally, specific pathogens have been identified as being able to block chemotaxis.

3. The phagocytic cell attaches itself to the pathogen cell. The pathogen cell can’t be absorbed by the phagocytic cell unless they are attached. There are two forms of attachment: enhanced attachment and unenhanced attachment.

   • Enhanced attachment relies on antibody molecules and complement proteins and it allows microbes to attach to the phagocytes. It is considered more specific and efficient compared to the unenhanced attachment.
   • Unenhanced attachment occurs when common pathogen-associated components that aren’t found in human cells are detected in the body. These components are found using receptors that live on the surface of phagocytes.

4. After the attachment, the phagocytic cell is ready to consume the pathogen. It absorbs the pathogen and a phagosome is formed. As the phagosome moves towards the center of the cell, a phagolysosome is formed. A phagolysosome is acidic and contains hydrolytic enzymes that help break down whatever was absorbed by the phagocytic cell.

5. Once the pathogen is broken down, it needs to be released by the phagocytic cell using a process called exocytosis. Exocytosis allows cells to remove toxins or waste from their interior.

What happens after phagocytosis occurs?

After phagocytosis occurs, dendritic cells (cells that help move T cells to antigens) are sent to one of the various organs in the body to present an antigen to a T cell in order for the T cell to recognise this antigen at a later time. This is known as antigen presentation.

This process also occurs with macrophages, a type of white blood cell that consumes other harmful cells.

Once phagocytosis is finished, exocytosis occurs. This means that cells are allowed to remove toxins from their interior.

Differences of pinocytosis and phagocytosis

Although phagocytosis helps take care of pathogens, pinocytosis is also helpful with destroying cells that can harm the body.

Instead of absorbing solids like phagocytosis, pinocytosis helps absorb liquids in the body. Pinocytosis typically ends up absorbing liquids such as ions, amino acids, and sugars. It is similar to phagocytosis in that small cells are attached to the outside of the cell then devoured. They also produce their version of a phagosome, known as a pinosome. Pinocytosis doesn’t use lysosomes like phagocytosis. It also absorbs all types of liquid and is not picky, unlike phagocytosis.