What is Phagocytosis?
Phagocytosis process is the process where a living cell engulfs other cells or particles. It is a form of endocytosis and the cell that carries the phagocytosis process is known as phagocyte. In some cells, phagocytosis can be a means of feeding such as amoeba and, in some cases, it can be an action mechanism of special types of cells such as white blood cells to protect the body from foreign organisms and particles and cellular debris. The cell or particle engulfed due to the phagocytosis mechanism is contained in a vesicle known as a phagosome. It is in this phagosome where the further process on the cell or the particle is carried out.
Given above is a simple phagocytosis definition Biology provides. As mentioned, the phagocytosis process takes place in two scenarios: (i) For feeding and, (ii) Protection against foreign bodies. A very crude and simple phagocytosis example displaying phagocytosis mechanism is that of a large oil drop joining with a small oil droplet in water. One big difference from this example is that the cell while carrying out phagocytosis, extends its cellular membrane into two protruding arms like structures, called pseudopodia, around the target cell or particle and then when the pseudopodia close the target cell or the target particle is engulfed inside the phagosome.
Inside the phagosome, the cell, cell debris or external particle is subjected to various kinds of treatment that lead to their degradation. The phagosome after the event of engulfing travels towards the centromere of the phagocyte and then fuses with the lysosomes. This new vesicle formed is known as phagolysosome. The hydrolytic enzymes present in lysosomes carry out the hydrolysis of the external particles leading to their degradation. After the digestion of the contents of the phagosome, the released nutrients are diffused or transported to the cytoplasm for use in different metabolic processes.
In the case of protists where phagocytosis mechanism is a mode of nutritional gathering, the nutrients are utilised by the phagocyte. But in cases of white blood cells, such as macrophages and neutrophils, where the phagocytosis process is a part of defense mechanism, the digested particles are used to further produce antibodies against specific parts of the foreign cell.
Stages of Phagocytosis
A simple phagocytosis diagram given below shows the phagocytosis mechanism. This phagocytosis diagram is a simple phagocytosis example of engulfing a bacterial cell and its destruction.
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The simple picture given above depicts various stages in which the phagocytosis mechanism can be divided. There are three major stages of phagocytosis divided as, particle adherence, engulfment and digestion. These stages are explained in detail below:
1. Particle Adherence
For the phagocytosis process to be successful it is quite important the target particle is engulfed or ingested inside the phagocyte. In special types of phagocytes such as macrophages, neutrophils, and dendritic cells which are essential for defending the human body against foreign invasion, the engulfing has the contribution to the further phagocytosis mechanism. Phagocytosis begins with the binding of the external body with the molecules known as ‘receptors’ present on the surface of the phagocytes. For special types of phagocytes which are part of the immune system, these receptors bind to opsonins which are protein components present in a film around the target cell or particle. Phagocytes adhere to these opsonins and then phagocytosis follows.
2. Particle Engulfment
After the receptors of the phagocytes bind to the protein films around the target particles, it stretches itself around the particle and engulfs it. Once, the particle is in the phagosome, the phagosome travels towards the centromere. Inside, the phagocyte, the phagosomes fuses with the lysosomes as already given above forming the phagolysosome. The time taken by the phagocyte for the engulfment varies depending on the particle size and the types of phagocytes. Inside the phagolysosome, the final stage of phagocytosis mechanism - the digestion begins where the bacterium or the particle is attacked by different types of killing machinery.
3. Particle Digestion
The hydrolytic enzymes present in the lysosomes when combined with the phagosomes, perform their hydrolysis on the bacterial cell or the particle. The digestive or the eating properties of the phagocyte is different for different types of phagocytosis. Macrophages for example are untidy eaters. They engulf huge quantities of external bodies and then release some undigested fractions back into the tissues. This is for the downstream signalling of the immune system for recruiting more phagocytes. Also, it can be different in case of adaptive immunity. But in case of protists such as amoeba the particles are digested and as explained above the nutrients are taken up by the phagocyte for its metabolic activities. Thus, the phagocytosis mechanism is essential for the survival of organisms either by procuring nutrients or defending against invading organisms.
One interesting fact about the different types of phagocytes is that even though they may be different they are voracious eaters. They can engulf huge amounts of external material. Scientists have tried to make the macrophages eat iron filings and then they have tried to separate the ones that have engulfed them using magnets.