Phagocytosis
Phagocytosis is a process used by some cells to take up large particles such as other cells. In this video you can see a macrophage phagocytosing a fungal spore. Don't you think it looks like it is eating the spore?
Phagocytosis plays an important role in pathogen removal by immune cells, but we must also remember that phagocytosis of dying or dead cells is an important everyday housekeeping activity of macrophages.
When we think of pathogens being eaten by immune cells it seems like the problem is solved just by physically removing the invader. But how do phagocytes like macrophages, neutrophils and dendritic cells know what to eat? Sometimes the signal can be recognition of a PAMP on the pathogen. In other cases a "warning flag" or opsonin, stuck to the pathogen can instruct the phagocyte to engulf the invader immediately. Can you identify what the "warning flag" is in this video?
Did you see how antibodies coated the bacteria? This is coating called opsonisation. Phagocytosis was started when the so called "Fc part" of the antibody bound to Fc-receptors on the phagocyte. Complement factors can also act as warning flags and start phagocytosis but in this case when binding to complement receptors. But how do phagocytes physically engulf pathogen and how is the pathogen destroyed? Go back to the video and see if you can figure this out. What is a pseudopod and phagosome? What does the lysosome do?
The video shows how the phagocyte extends long tube-like protrusions around the antibody-coated pathogen but the way in which pathogens are eaten can vary. For example, complement-opsonised invaders seem to sink into the immune cell, like getting on a very large, soft waterbed. Phagocytosis allows for "safe" disposal of the pathogen. The lysosomes deliver chemicals and enzymes that kill and digest the pathogen far away from other cells that could get damaged or infected. But how then do surrounding cells know about the threat? Well macrophages and dendritic cells alert the adaptive immune system by showing small digested parts of the pathogen in a process called antigen presentation, and all phagocytes release inflammatory cytokines that alert surrounding cells as seen here.
Notice how the other immune cells are rushing to the macrophage. Why and how do they do that and what might happen next?