Phagocytosis Process and Role in Immune Defense

What is Phagocytosis?

Phagocytosis is when certain living cells called phagocytes use their plasma membrane to eat or engulf other large cells or particles (≥ 0.5 μm), creating an internal chamber named the phagosome. It is a variety of endocytosis. The cell that does phagocytosis is termed a phagocyte. And when a phagocytic cell exposed to an opsonized surface and spreads itself in a manner where it tries to engulf it, it is called frustrated phagocytosis.

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The phagocytic cell can be any one of the body cells like a phagocytic white blood cell or a one-celled organism like an amoeba. While phagocytosis in amoeba and some other primate life classes, such as sponges, is referred to as the feeding mechanism. And in some higher primates, phagocytosis is essentially a preventive reaction to germ and body incursion by foreign materials (antigens). However, epithelial and endothelial cells are not utilized for the purpose as they are examples of non-phagocytic cells.

Early Observations

The appearance of foreign bits within cells was originally described by a pathologist named Kranid Slavjansky in the 1860s. Later a Russian-born microbiologist and zoologist, Élie Metchnikoff, in the 1880s pronounced the term phagocyte, wherein immune cells engulf and kill foreign bodies like bacteria. He also noticed that phagocytic cells perform a significant immune response function and observed the phenomena of frustrated phagocytosis. This discovery even drew a Nobel Prize of 1908 to him for Physiology.

Types of Phagocytosis

There are three principal types of phagocytosis:

1. Macrophages

2. Monocytes

3. Dendritic Cells

4. Granulocytes

Macrophages

Macrophage phagocytosis is a type of phagocytosis which is obtained from monocytes and is observed in the tissues. Macrophage phagocytosis has a significant role as a primary defense mechanism in the phagocytosis of cellular wastes, microorganisms, and different foreign objects.

Monocytes

Monocytes are a variety of phagocytic cells that are located in the bloodstream. They flow throughout the body, and when the tissue is contaminated, it might leave the bloodstream and invade the tissue.

Dendritic Cells

The main objective of dendritic cells is to work as an attachment between the adaptive immune system and the innate. As young dendritic cells travel in the bloodstream, move through tissues, and continuously sample the pathogens (via phagocytic white blood cells who are accountable) they encounter via macr,opinocytosis.

Granulocytes

Granulocytes are a collection of phagocytes and dense granules in their cytoplasm, including eosinophils, basophils, and neutrophils.

Process of Phagocytosis 

Living cells take in diverse types of substances to thwart their cell membrane. Most of these molecules or materials are fluids, ions, and oxygen, while others easily cross through the membrane by mechanisms like osmosis and ion pumps.

Some objects, such as particles like viruses, might prove too big to cross through the sheath by such mechanisms. This is why the cell begets to engulf so much matter/objects inside the cell. 

This method includes the introversion of the cell membrane, which allows the cell to carry in the particle. Depending on the cell and technique used to submerge such objects, endocytosis is classified into pinocytosis, phagocytosis and an added method perceived as receptor-mediated endocytosis. 

What separates phagocytosis from pinocytosis is the possession of special surface proteins in phagocytes that empower them to recognize and join to settled particles before engulfing them. This variety of endocytosis is reliant on the junction connecting the cell and the aim object.

Steps of Phagocytosis

There are four steps of phagocytosis, namely:

Step 1: Actuation or activation

This step of phagocytosis transpires when the cell grows in close contiguity to presented objects. For example, when phagocytes are close to such cells as bacteria, this stimulates the phagocytes and spurs binding.

NOTE: The initial step of phagocytosis also requires chemotaxis. Here, the cells progress towards the region with large foreign particles or molecules concentration. Cells are chemically aroused by the closeness of the foreign particles etc.

Step 2: Coupling or binding

In this step, covering receptors upon the phagocyte adhere to the covering of the object. This step is essential for the particle to be engulfed.

Varying kinds of cover receptors play a significant role in phagocytosis (binding) depending on the cell.

Those include:

• Antibodies- Few cells are competent in creating antibodies that make it probable to connect to some antigens.

• Scavenger receptors- Connect to different types of molecules on the cover of bacteria.

• Toll-like receptors- Certain receptors bind to particular molecules on the cover of bacteria, fungi, and viruses; toll-like receptors are those. 

• Opsonin receptors- These are some of the commonly studied cover receptors. These bind to particles that control immunoglobulin G on their cover.

NOTE: Receptor junction is imperative in phagocytosis, permitting the cell to recognize the object, etc., and thus obtain the proper reaction.

Step 3: Ingestion

The phagocyte starts unfolding as it encircles the particle. This process of phagocytosis also includes the creation of a vesicle or vacuole around the particle as it is totally ingested.

Step 4: Conversion or digestion

In some cells, enzymes in the vesicle-like lysosome break down the particle into single components. Excess materials that cannot be utilized are later separated from the cell through exocytosis. Nevertheless, for phagocytes required in immunization, special formations known as peroxisomes are formed to confine and eliminate toxic particles.

Did You Know?

• When mammalian sperm cells infiltrate the female genital expanse, the analysis of the separation of the sperm by different enzymes are termed as phagocytosis of sperm.

• Neutrophil phagocytosis is the ingest and subsequently kill attacking microbes.

• When a phagocytic cell fails to extend itself to enclose large asbestos, it is called frustrated phagocytosis.

• Phagocytic white blood cells are accountable for the discharge of pathogens.

• Phagocytosis in amoeba and some other primate life classes, such as sponges, is referred to as the feeding mechanism. 

• Epithelial and endothelial cells are non-phagocytic cells examples.