immune system cells

• If a pathogen breaches all of these barriers they are able to enter the blood stream. At this point cells within the Immune System work together to try to prevent further spread. Phagocytes are specialised white blood cells which engulf and destroy pathogens
• There are 2 main phagocytes -

  • neutrophils
  • macrophages
• Dependent on their function, the different types of blood cell will have different adaptations. This makes them all recognisable on blood smears and films because of their characteristic features.
• neutrophils -

  • They have large amounts of Lysosomes, which contain enzymes that kill ingested cells. This makes their Cytoplasm appear Granular.
  • They also have a Lobed Nucleus, which allows for greater flexibility.

  Around 70% of white blood cells are neutrophils. They build up at a site of infection- usually forming pus
• Macrophages are specialised white blood cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms.
• Macrophages develop in the bone marrow as monocytes. Macrophages are given different names depending on the type of tissue they are found in.

  • red pulp - spleen
  • kupffer cells - liver
  • microglia - central nervous system
  • osteoclasts - bone
• Macrophages are able to detect products of bacteria and other microorganisms using recognition receptors such as Toll-like receptors (TLRs).

  These receptors can bind specifically to different pathogen components like sugars, RNA, DNA or extracellular proteins (for example, flagellin from bacterial flagella)

  Macrophages can also present antigens to T cells and initiate inflammation by releasing molecules (known as cytokines) that activate other cells
• Once phagocytes like Macrophages engulf a pathogen they produce chemicals known as cytokines. These essentially act as cell signallers.

  They can:

  • Stimulate the hypothalamus to increase body temperature
  • Signal to other phagocytes that they need to help with the attack

  There are many types of cytokine, including chemokines, interferons and interleukins
• Opsonins are extracellular proteins which bind directly to the pathogen attacking the body

  They essentially “tag” the pathogen to phagocytes. 

  Phagocytes then recognise these opsonins as they have receptors to them on their cell membrane.

  Once binding to these opsonins, the phagocytes then know to engulf the pathogen.
• Macrophage - A phagocyte involved in phagocytosis. Engulfs and destroys pathogens. Forms from monocytes.
• neutrophil - The most prevalent white blood cell. A phagocyte which has adapted for its function by having many lysosomes
• opsonin - bind directly to the pathogen attacking the body tagging or marking them.
• cytokine - Cell signalling chemicals released by phagocytes which signal to other immune cells to help.
• 1. Pathogens secrete chemicals which can attract phagocytes such as neutrophils and macrophages.
  2. Pathogens have proteins on their surface called antigens. These will be recognised by phagocytes as “non-self”
  3. the phagocyte then binds to the pathogen.
  4. the phagocyte then engulfs the pathogen, enclosing it within a structure called a phagosome
  5. Lysosomes within the phagocyte migrate towards the phagosome. They combine with the phagosome forming a phagolysosome where lysosome enzymes break down and digest the pathogen.
  6. In neutrophils this takes 10 minutes. In macrophages there is an extra step.
• • When macrophages digest the pathogen, they take the antigens from the pathogen they have engulfed and they mix them with glycoproteins in their cytoplasm called the major histonecompatability complex
  • This HPC then presents the pathogen’s antigens on the cell surface of the macrophage= antigen presenting cell
  • Stimulate cells within the specific immune response