cell recognition and the immune system

• what is a pathogen?

  an microorganism that causes disease
• what is immunity?


  when the body protects itself from infection and the reason why some people are unaffected by pathogens
• what is a non-specific mechanisms?

  it is the first line of defense in the immune system, providing a general, immediate response to pathogens. These mechanisms do not distinguish between different types of pathogens—they respond the same way to all invaders.
• what are non-specific mechanisms examples?

  • physical and chemical barriers
  • phagocytosis
• what are examples of physical and chemical barriers?

  • skin - acts as a physical barrier, preventing pathogen entry
  • mucus - traps pathogen in the respiratory tract
  • stomach acid - kills pathogens ingested with food or drink
• what do phagocytosis do in the non-specific mechanisms?

  If pathogens bypass barriers and enter the body, phagocytes (a type of white blood cell) play a role in engulfing and destroying them.
• why is it called non-specific?

  • These mechanisms do not rely on recognising specific antigens.
  • They respond the same way to all pathogens
• what are specific mechanisms?

  it is the second line of defence in the immune system. involves a targeted response against particular pathogens based on their unique antigens.
• what are the key features of specific mechanisms?

  • response is less rapid
  • long-lasting immunity
  • involves T and B Lymphocytes
  • creates memory cells
• what are neutrophils?


  • found in blood and tissue fluid
  • made in bone marrow
  • they are short-acting and initiate the immediate response
• what are macrophages?


  • found in blood and lymph nodes
  • bigger than neutrophils
  • macrophages develop in lymph nodes and are made in bone marrow
  • macrophages are involved in adaptive, specific immune respone
• what is the first step of phagocytosis?

  • Chemotaxis
  • phagocytes are attracted to the site of infection by chemical signals released by pathogens or damaged tissues
• how do phagocytes recognise and attach to pathogens?

  Phagocytes have pathogen recognition receptors (PRRs) that recognise and bind to unique antigens called pathogen-associated molecular patterns (PAMPs) on the cell surface membrane of pathogens.
• what happens during the engulfment stage of phagocytosis?

  The phagocyte grows cytoplasmic extensions to engulf the pathogen and internalises it into its cytoplasm. The pathogen is engulfed in a vesicle known as the phagosome. 
• what occurs when the phagosome fuses with a lysosome?

  the phagosome fuses with a lysosome forming a phagolysosome. The lysosome releases lysozyme into the phagosome which then hydrolyse the pathogen
• what happens inside the phagolysosome?

  • digestion
  • digestive enzymes from the lysosome break down the pathogen's components, neutralising them.
• what occurs after digestion of the pathogen?

  • exocytosis
  • the digested products are either expelled through exocytosis or displed on the phagocyte's surface to trigger further immune responses
• what are antigens?

  Foreign substances that trigger an immune response and production of an antibody
• what are lymphocytes?

  type of white blood cell produced by stem cells in the bone marrow
• what are the 2 types of lymphocytes called?

  • B lymphocytes (B cells)
  • T lymphocytes (T cells)
• what are B lymphocytes?

  • mature in bone marrow
  • associated with humoral immunity (immunity involving antibodies that are present in the body fluids)
• what is T lymphocytes?

  • mature in the thymus gland
  • associated with cell-mediated immunity (immunity involving body cells)
• what is the process of cell-mediated immunity?

  1. pathogens invade body cells or are taken in by phagocytes
  2. the phagocytes place antigens from the pathogen on its cell-surface membrane
  3. receptors on a specific helper T cell fits exactly onto these antigens
  4. this activated the T cell to divide rapidly by mitosis and form a clone of genetically identical cells
  5. cloned T cells:
  6. memory cells
  7. stimulate phagocytes
  8. stimulate B cells to divide
  9. activate cytotoxin T cells