Phagocytosis

Phagocytosis is a fascinating process used by cells to engulf and consume large particles. The word comes from Ancient Greek, combining φαγεῖν (phagein) meaning "to eat" and κύτος (kytos) meaning "cell." During phagocytosis, a cell uses its plasma membrane to surround and take in particles that are at least 0.5 micrometers in size. This forms a special compartment inside the cell called a phagosome. Phagocytosis is one way that cells perform a larger process known as endocytosis.

Many living things use phagocytosis for important jobs. In the immune system of multicellular organisms, special cells called phagocytes eat away harmful pathogens and clear up dead cells. After bringing these particles inside, the cell can break them down and use the pieces for energy or building new parts. Everyday examples of things that can be eaten this way include bacteria, bits of dead tissue, and even tiny mineral particles.

Some single-celled creatures, like certain protozoa, rely on phagocytosis to get their food. Two key players in the human immune system are macrophages and neutrophils, both of which use this process to defend the body. When phagocytosis is used mainly for getting food rather than fighting disease, it is called phagotrophy. This is different from another way cells get food called osmotrophy, where nutrients are taken in directly through the cell’s surface by absorption.

History

The history of phagocytosis shows how scientists first learned about the immune system. In 1849, a Swiss scientist named Albert von Kölliker described how a tiny alga could swallow small organisms, an early look at how cells take in materials.

Later, in 1862, a German scientist named Ernst Haeckel showed that blood cells from a sea slug could take in particles like Indian ink, giving the first proof that immune cells could do this. The process was later studied and named by Élie Metchnikoff in the years 1880 and 1883.

In immune system

Main article: Phagocyte

Phagocytosis is an important part of the body’s natural defense against infections. It helps the immune system respond to germs and start a more specific defense. While many cells can perform phagocytosis, some types do it as their main job. These are called professional phagocytes and include cells like neutrophils, macrophages, and dendritic cells.

Professional phagocytes work in different ways. Neutrophils move quickly to areas of infection and swallow up germs. Macrophages stay in tissues and can continue to engulf germs for a long time. Dendritic cells also capture germs but mainly help by showing pieces of them to other immune cells to start a stronger response.

In apoptosis

After a cell dies in a controlled way, known as apoptosis, it needs to be cleared away by special cells in the body called macrophages. This process is called efferocytosis. Apoptotic cells show certain molecules on their surface, like calreticulin and phosphatidylserine, which help macrophages recognize and engulf them. If this clearance does not happen properly, it can lead to problems such as autoimmune disorders. Enhancing phagocytosis might help treat some of these conditions.

Main article: Efferocytosis

In protists

Many tiny living things called protists use phagocytosis to eat. This means they surround and take in food particles using parts of their cell. For example, amoeba can wrap around things using special arm-like structures called pseudopods, similar to how some cells in our bodies work. Another group of protists, called ciliates, have a special groove or opening in their cells called the cytostome where they take in food.

After the protist brings in food, it mixes this with special parts of the cell that have digestive chemicals. This helps break down the food so the cell can use the nutrients for energy and growth. Some protists can even mix this way of eating with getting energy from sunlight.