Phagocytosis

• Macrophages are the most abundant type of white blood cell in the body and are involved in phagocytosis. They sit in lymphnodes in the lymphatic system, which swell when phagocytosis is occurring.
• Chemotaxis is the movement of cells to a chemical stimulus. For example, a bacterium releases chemicals as it moves; the phagocyte recognises these and begins to chase it down. The phagocyte moves up the concentration gradient towards the bacterium.
• "Chemo" = chemical

  "Taxis" = to move

  "Chemotaxis" = to move following chemicals
• Once macrophages have hydrolysed a bacterium, they push an antigen (taken from the bacteria) through their cell membrane to communicate the presence of the bacterium to the rest of the immune system.

  This macrophage is now called an antigen-presenting cell (APC).
• Phagocytes are made in the bone marrow, travel in capillaries and can squeeze through capillary walls into tissues. They patrol the body, searching for antigens. There are two types: neutrophils and macrophages.
• Neutrophils engulf and digest pathogens/dead human cells/debris. They are short lived and die once they have engulfed antigens. These are the phagocytes that go to the site of local infection.
• Macrophages are larger (macro) phagocytes which live longer and tend to circulate in the lymphatic system.
• Phagocytosis is the cellular process carried out by phagocytes of engulfing solid particles using the cell membrane.
• Phagocytosis:

  1. the pathogen is recognised as having non-self antigens; the phagocyte is attracted to the pathogen by chemicals released by the pathogen in a process called chemotaxis, and the phagocyte attaches to the pathogen by surface receptors
  2. the pathogen is engulfed by the phagocyte by endocytosis, forming a phagosome
  3. lysosomes fuse to the phagosome, releasing digestive enzymes into the phagosome to digest the pathogen
  4. harmless products are excreted by exocytosis or used by the phagocyte
• A phagosome is formed when a phagocyte engulfs a pathogen and its membrane fuses around it. Lysosomes fuse to this and release hydrolytic enzymes into it to digest the pathogen.
• Phagocytosis is a non-specific response that is sometimes not enough to cope with large numbers of pathogens e.g. the flu virus. Phagocytes that engulf the virus present some of the pathogen's antigens on their cell membrane - they become antigen-presenting cells (APCs). This activates T-helper cells, which stimulate and recruit more phagocytes/T-cells to assist in the immune response. This includes cytotoxic (killer) T-cells which destroy infected/foreign cells by releasing chemicals (e.g. perforin) into the invaded cell.
• Cytotoxic (T𝒸) cells bind to the APC body cell. It uses an external protein to bind to the APC cell surface. It then releases a substance called perforin, which perforates (makes pores in) the infected cell's membrane. The body cell falls apart and dies, no longer able to produce viruses.
• Each cytotoxic T-cell (T𝒸) has a specific receptor and so will only bind to one specifically-shaped antigen. A different T𝒸 is needed for each different antigen.
• Some white blood cells are phagocytic. How do these phagocytic white blood cells destroy bacteria?

  1. Phagocyte attracted to bacteria by chemicals (chemotaxis)/recognise antigens on bacteria as foreign;
  2. Engulfs/ingests bacteria;
  3. Bacteria in phagosome/vesicle;
  4. Lysosome fuses with / empties hydrolytic enzymes into phagosome;
  5. Bacteria digested/hydrolysed;
• How are phagocytes and lysosomes involved in destroying microorganisms?

  • Phagocytes engulf pathogens/microorganisms;
  • Enclosed in a vesicle/phagosome;
  • Fuses to a lysosome with hydrolytic enzymes inside;
  • That digest/hydrolyse microorganisms;
• The variable region of the antibody is specific to the shape of one antigen on a bacterium.
• 𝙋𝙃𝘼𝙂𝙊𝘾𝙔𝙏𝙊𝙎𝙄𝙎:

  1. The phagocyte detects the pathogen by chemotaxis (release of chemicals).
  2. The pathogen binds to the cell surface receptors on the phagocyte.
  3. The pathogen is engulfed by the phagocyte.
  4. The pathogen becomes enclosed in a phagosome.
  5. The phagosome fuses with a lysosome containing hydrolytic enzymes.
  6. The hydrolytic enzymes are released into the phagosome, digesting/hydrolysing the pathogen.
• Scientists use an antibody to detect an antigen on the bacterium that causes stomach ulcers. Explain why the antibody will only detect this antigen.

  1. Antibody/variable region has specific amino acid sequence/primary structure;
  2. The shape/tertiary structure of the binding site;
  3. Is complementary to/fits/binds with these antigens;
  4. Forms complex between antigen and antibody;
• Describe how a phagocyte destroys a pathogen present in the blood.

  The phagocyte engulfs the pathogen to form a phagosome, which fuses with a lysosome containing hydrolytic enzymes which digest/hydrolyse the pathogen.
• What is the role of the disulphide bridge in forming the quaternary structure of an antibody?

  Joins two polypeptides.
• Antibody diagram:

  A) Constant

  B) Heavy chain

  C) Disulphide bridge

  
• Both neutrophils and macrophages carry out phagocytosis.
• The phagocytotic cell engulfs the pathogen with protruding arms called 'pseudopodia'. The membranes of these arms fuse together to enclose the pathogen in a vacuole called a phagosome.
• Lysosomes fuse with the phagosome to produce a phagolysosome. The lysosome empties hydrolytic enzymes into the phagosome, including lysozyme which hydrolysis bacterial cell walls.
• A phagosome is a small, membrane-bound vesicle containing a pathogen.
• A phagolysosome is a phagosome fused with a lysosome.
• Lysozyme in lysosomes hydrolyses the murein (peptidoglycan) in bacterial cell walls, allowing other hydrolytic enzymes to digest the rest of the pathogen.
• Phagocytes are white blood cells that carry out phagocytosis.
• Phagocytes are made in the bone marrow, are trained in the thymus gland, and travel through the capillaries into body tissues. They partol the body, searching for antigens.
• Macrophages are large phagocytes, are long-lived, and circulate in the lymphatic system. They are often involved in the larger immune response.
• Neutrophils are smaller, short-lived phagocytes which die after engulfing antigens. They go to the site of local infection.
• What is phagocytosis?

  A cellular process of engulfing solid particles using the cell membrane. It is carried out by phagocytes.
• 𝙋𝙃𝘼𝙂𝙊𝘾𝙔𝙏𝙊𝙎𝙄𝙎:

  1. A pathogen is recognised as having non-self antigens. The phagocytes is attracted by chemicals released by the pathogen (chemotaxis).
  2. The pathogen attaches to the phagocytes via surface receptors and is engulfed (endocytosis), forming a vesicle/phagosome.
  3. Lysosomes fuse to the phagosome (phagolysosome) and release lysozyme into it to hydrolyse the pathogen.
  4. Harmless products are used by the phagocytes or are egested by exocytosis.
• Describe how phagocytosis of a virus leads to presentation of its antigens.

  1. Phagosome fuses with lysosome;
  2. Virus is destroyed by lysozymes;
  3. Antigens from virus are displayed on the cell membrane;
• Describe how the presentation of a virus antigen leads to the secretion of an antibody against the virus antigen.

  1. Helper T cell binds to the antigen on the antigen-presenting cell;
  2. This helper T cell stimulates a specific B cell;
  3. B cell clones;
  4. Forms plasma cells that release antibodies;