Immunity

• What is an antigen?

  ● Foreign molecule / protein

  ● That stimulates an immune response leading to production of antibody
• How are cells identified by the immune system?

  ● Each type of cell has specific molecules on its surface (cell-surface membrane / cell wall) that identify it

  ● Often proteins → have a specific tertiary structure
• What types of cells and molecules can the immune system identify?

  1 Pathogens (disease causing microorganisms)eg.viruses, fungi, bacteria

  2 Cells from other organisms of the same species (eg. organ transplants)

  3 Abnormal body cells eg. tumour cells or virus-infected cells

  4 Toxins (poisons) released by some bacteria
• Describe phagocytosis of pathogens (non-specific immune response)

  1 Phagocyte attracted by chemicals on pathogen

  2 Phagocyte engulfs pathogen by surrounding it with its cell membrane

  3 Pathogen contained in phagosome in cytoplasm of phagocyte

  4 Lysosome fuses with phagosome and releases lysozymes

  5 Lysozymes hydrolyse pathogen
• Describe the response of T lymphocytes to a foreign antigen (the cellular response)

  T lymphocytes recognise antigen presenting cells (eg. infected cells, phagocytes presenting antigens, transplanted cells, tumour cells etc.)

  Specific helper T cells with complementary receptors bind to antigen on APC → activated and divide by mitosis to form clones which stimulate:

  ● Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin)

  ● Specific B cells (humoral response - see below)

  ● Phagocytes → engulf pathogens by phagocytosis
• Describe the response of B lymphocytes to a foreign antigen (the humoral response)

  B lymphocytes can recognise free antigens (eg. in blood or tissues,) not just antigen presenting cells.

  1. Clonal selection:

  ● Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen

  ● This is then stimulated by helper T cells

  ● So divides rapidly by mitosis to form clones

  2. Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody

  3. Some differentiate into B memory cells → remain in blood for secondary immune response
• What are antibodies?

  ● Quaternary structure proteins

  ● Secreted by B lymphocytes (eg. plasma cells) in response to specific antigens

  ● Bind specifically to antigens forming antigen-antibody complexes
• Describe the structure of an antibody

  Antigen

  antigen binding site

  variable region

  disulfide bridge

  constant region

  Light chain

  heavy chain
• Explain how antibodies lead to the destruction of pathogens

  ● Antibodies bind to antigens on pathogens forming an antigen-antibody complex

  ○ Specific tertiary structure so binding site / variable region binds to complementary antigen

  ● Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens

  ● Antibodies attract phagocytes

  ● Phagocytes bind to the antibodies and phagocytose many pathogens at once
• Explain the differences between the primary & secondary immune response

  ● Primary- first exposure to antigen

  ○ Antibodies produced slower & at a lower conc.

  ○ Takes time for specific B plasma cells to be

  stimulated to produce specific antibodies

  ○ Memory cells produced

  ● Secondary- second exposure to antigen

  ○ Antibodies produced faster & at a higher conc.

  ○ B memory cells rapidly undergo mitosis to

  produce many plasma cells which produce

  specific antibodies