The Immune System: Phagocytosis | A-level Biology | OCR, AQA, Edexcel

The Immune System: Phagocytosis in a Snap! Unlock the full A-level Biology course at bit.ly/2Jy9UbA created by Adam Tildesley, Biology expert at SnapRevise and graduate of Cambridge University.
The key points covered of this video include:
1. Types of Phagocytes
2. Mechanisms of Phagocytosis
3. Phagocyte Specialisations
Types of Phagocytes
Some cells in the body can engulf and digest pathogens to stop them causing damage. This process is called phagocytosis and any cell that does this is called a phagocyte. Example of phagocytes include macrophages and neutrophils. Monocytes are precursors to macrophages. Both monocytes and neutrophils are made in the bone marrow. Monocytes are present in the blood when they enter the tissues they become macrophages. When any pathogen infects a tissue, neutrophils arrive first and each neutrophil can engulf 5-20 pathogenic cells. Neutrophils die quickly after a few days whereas macrophages are long lived cells. When neutrophils die, the macrophages then arrive at the infected tissue and each macrophage can engulf 100 pathogenic cells.
Mechanisms of Phagocytosis
During phagocytosis, the phagocyte first recognises a foreign marker on the pathogen’s outer membrane called the antigen. Small non-specific protein molecules called opsonins then attach to the antigen. The phagocyte can then bind to the opsonins attached to the pathogen antigen - allowing the phagocyte to get closer to the pathogen. The pathogen is then engulfed and enclosed in a large vacuole called a phagosome. This phagosome then fuses with lysosomes which are vesicles containing digestive enzymes called lysozymes. This causes the lysozymes to be released and they break down the pathogen. There are a few special cells like macrophages that do not completely destroy the pathogen and instead save the pathogen antigen. These cells put the pathogen antigen on a special protein complex. This is then moved to the cell surface membrane so other immune cells can recognise the pathogen antigen. Macrophages and other special cells which do this are called antigen presenting cells.
Phagocyte Specialisations
Phagocytes are specialised to carry out each step in phagocytosis. Phagocytes have well developed cytoskeletons to help them change shape to engulf the pathogen and move lysosomes around. They have many mitochondria to release energy required for cell movement. They also contain lots of ribosomes to synthesise the lysosome enzymes. Phagocytes have a lobed nucleusto help them squeeze throught narrow gapsbetween cells in the tissues.
Summary
Macrophages are long-lived phagocytes and neutrophils are short-lived phagocytes
Phagocytes recognise foreign antigens on pathogens
Opsonins bind to the antigen to help phagocytes get closer to the pathogen
During phagocytosis, the pathogen is first engulfed into a phagosome
Lysosomes fuse with the phagosome and this kills the pathogen
In macrophages, which are antigen-presenting cells, the pathogen antigen is saved and presented on the surface of the cell
Phagocytes are specialised to help them carry out phagocytosis

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[@jimcoote5094]

Some of the information is inaccurate or misleading 1) I would not describe antibodies bound to bacterial antigens as 'non-specific'. They are of course highly specific. I presume that the 'non-specific' term was meant to describe non-specific opsonins such as C3 that often speed up and amplify the phagocytic response. That's fine, but I would change the video making it clear that the antibodies themselves are highly specific in their binding, (the most specific in all nature probably!). 2) The digestive enzymes within lysosomes are NOT all called lysozymes. Lysozyme IS found within lysosomes but is quite different from most of the other enzymes found there. One of its main functions is to breakdown peptidogylcans of bacteria that are outside the cell, which is why so much is secreted. Good video, but tighten things up a bit.

[@mariamassi9617]

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