The immune system

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The immune system has non-specific and specificcresponses to pathogens. Auto-immune diseases are the result of failures in the system to distinguish between self and non-self.

1. State that phagocytes (macrophages and neutrophils) have their origin in bone marrow and describe their mode of action

  • Phagocytes are produced in the bone marrow
  • They are stored there before being distributed throughout the entire body in the blood
  • They are scavengers, removing any dead cells as well as invasive microorganisms
  • Neutrophiles
    • They are agranulocyted phagocytes and they travel throughout the body by squeezing through the walls of blood capillaries to 'patrol' the tissues
    • During an infection, neutrophils are released in large numbers 
    • Histamine is released by cells under attack and these, with any chemicals released by the pathogens themselves, attract passing neutrophils
    • This movement towards a chemical stimulus is called chemotaxis
    • The neutrophils move towards the pathogens and the antibodies which cover the clustered pathogens further stimulates the neutrophils to attack the pathogens
    • When the neutrophil attaches to the pathogen, the neutrophil's cell surface membrane engulfs the pathogen and traps it within a phagocytic vacuole in a process called endocytosis
    • Digestive enzymes are secreted into the vacuole so the pathogen is destroyed
    • After neutrophils kill and digest some pathogens, they die so they have a short life
  • Macrophages
    • They are granulated phagocytes and are larger than neutrophils 
    • They tend to be found in organs such as the kidney and lymph nodes rather than remaining in the blood
    • Macrophages travel in the blood as monocytes after they are produced in the bone marrow
    • They only develop into macrophages once they settle into the organs
    • They are long-lived cells and play a crucial role in initiating the immune responses
    • They do not completely destroy pathogens, but cut them up to display the antigens that can be recognised by the lymphocytes

2. Describe the modes of action of B-lymphocytes and T-lymphocytes

  • B-lymphocytes (B cells) are in charge of the humoral response
    • They remain in the bone marrow until they mature then they move around the body, concentrating in the lymph nodes and the spleen
    • When an antigen enters the body for the first time, the small numbers of B cells with receptors complementary to the antigen are stimulated to divide by mitosis in the clonal expansion stage
    • Some of these activated B cells become plasma cells and produce antibody molecules very quickly
    • Plasma cells secrete antibodies into the blood or lymph and these plasma cells do not live long
    • Other B cells become memory cells 
    • These cells remain circulating in the body for a long time so if the same antigen is reintroduced, memory cells divide rapidly and develop into plasma cells and more memory cells
  •  T-lynphocytes (T cells) are responsible for the cell mediated response
    • The leave the bone marrow and collect at the thymus when they mature
    • Mature T cells have specific cell surface receptors called T cell receptors
    • T cell receptors have a structure that are similar to that of antibodies and they are each specific to one antigen
    • T…


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