Cell recognition and the immune system

• Antigen

  Any part of an organism or substance that is recognised by the immune system as non-self and which stimulates an immune response
• Pathogen

  Organisms that cause disease. All pathogens have antigens on their surface
• PHAGOCYTOSIS

  1. Phagocyte recognises the foreign antigens on pathogen

  2. Cytoplasm of phagocyte engulfs pathogen

  3. Pathogen is contained in phagocytic vacuole in cytoplasm of phagocyte

  4. Lysosomes containing hydrolytic enzymes (lysozymes) fuse with the phagocytic vacuole. Lysozymes break down pathogens

  5. Phagocytes present the pathogens antigens on its surface to activate the immune system. Phagocytes act as antigen presenting cells
• T CELLS

  Type of white blood cell. Has receptor proteins on its surface that bind to the complimentary antigens - causes activation of T-cell. Helper T-cells release chemical signals → activate and stimulate phagocytes, cytotoxic T-cells - which kill abnormal and foreign cells. Also activate B-cells
• B CELLS

  Type of white blood cell. Covered with antibodies (proteins that bind to antigens to form an antigen-antibody complex. Each B-cell has a different shaped antibody. When antibody binds with complimentary shaped antigen, along with substances released from T-cells, activates B-cells - CLONAL SELECTION. Activated B-cells divide into plasma cells
• NON SPECIFIC IMMUNITY

  • Include physical barriers and phagocytosis
  • Physical barriers: Mucus membranes trap pathogens, Dead skin cells block pathogens, Hydrochloric acid kills pathogens in the stomach
• SPECIFIC IMMUNITY

  • Include cell mediated response and humoral response
• Humoral response

  1. Surface antigens of invading pathogens are taken up by B cells

  2. B cells process the antigens and present them on their surface

  3. Helper T cells attach to the antigen presenting cells which activate the B cell

  4. B cell divides by mitosis and creates plasma cells

  5. Cloned plasma cells secrete antibodies that are specific to the antigens of surface of the pathogen

  6. Antibodies attach and destroy the pathogen

  7. Some B cells may divide into memory cells which can then trigger a secondary immune response
• Cell mediated response

  1. Helper T cells mature in the thymus glands

  2. T cells respond to antigen presenting cells such as cancer cells, infected body cells, or macrophages

  3. Receptors on specific helper T cells are complementary to these antigens

  4. Attachment triggers division of T cells through mitosis and form clones of genetically identical cells

  5. Cloned cells can develop into memory cells, stimulate phagocytosis, stimulate B cells to divide and secrete antibodies, and activate cytotoxic T cells