Phagocytosis

Phagocytosis is a way that cells can take in large pieces of material. The word comes from Ancient Greek, joining φαγεῖν (phagein) meaning "to eat" and κύτος (kytos) meaning "cell." In this process, a cell uses its plasma membrane to wrap around and bring in particles that are at least 0.5 micrometers big. This makes a special space inside the cell called a phagosome. Phagocytosis is one type of a bigger process called endocytosis.

Many living things use phagocytosis for important work. In the immune system of animals, special cells called phagocytes can eat harmful pathogens and clean up dead cells. After bringing these inside, the cell can break them apart and use the pieces for energy or to build new parts. Common things eaten this way include bacteria, small bits of dead tissue, and tiny mineral pieces.

Some tiny creatures, like certain protozoa, use phagocytosis to find their food. Two important cells in the human immune system are macrophages and neutrophils, both of which use this process to help protect the body. When phagocytosis is used mainly for finding food instead of fighting sickness, it is called phagotrophy. This is different from another way cells get food called osmotrophy, where nutrients are taken in straight through the cell’s surface.

History

The history of phagocytosis shows how scientists learned about the immune system. In 1849, a Swiss scientist named Albert von Kölliker saw that a tiny alga could take in small organisms. This was 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. This was the first proof that immune cells could do this. The process was studied and named by Élie Metchnikoff in 1880 and 1883.

In immune system

Main article: Phagocyte

Phagocytosis is a key way the body fights off infections. It helps the immune system react to germs and start a more focused defense. Many cells can do phagocytosis, but some do it most of the time. These are called professional phagocytes and include cells like neutrophils, macrophages, and dendritic cells.

Professional phagocytes work in different ways. Neutrophils move fast to places with infection and take in germs. Macrophages stay in tissues and can keep taking in germs for a long time. Dendritic cells also catch 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, called apoptosis, special cells in the body called macrophages clean it up. This process is called efferocytosis. Apoptotic cells show certain molecules on their surface, like calreticulin and phosphatidylserine, which help macrophages recognize and remove them. If this cleanup does not happen properly, it can cause problems such as autoimmune disorders. Improving 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. 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 also get energy from sunlight.