Cell Recognition and the Immune System

• What is an antigen?

  A cell-surface molecule that stimulates an immune response.
• What types of molecules can serve as antigens?

  Antigens can be (glyco)proteins, (glyco)lipids, or polysaccharides.
• How does the immune system recognize antigens?

  It recognizes them as "self" or "non-self," enabling identification of various cells.
• What does the immune system identify using antigens?

  It identifies cells from other organisms, pathogens, toxins, and abnormal body cells.
• What is the first step in the process of phagocytosis?

  The phagocyte moves towards the pathogen via chemotaxis.
• What happens after a phagocyte engulfs a pathogen?

  A phagosome is formed through endocytosis.
• What occurs when a phagosome fuses with a lysosome?

  A phagolysosome is formed.
• What is the role of lysozymes in phagocytosis?

  Lysozymes digest the pathogen.
• What does the phagocyte do after digesting the pathogen?

  The phagocyte absorbs the products from pathogen hydrolysis.
• What is the role of antigen-presenting cells (APCs)?

  APCs display antigens from pathogens on their surface after hydrolysis.
• How do APCs enhance recognition by T helper (TH) cells?

  They present antigens that TH cells cannot directly interface with in body fluid.
• What are the two types of immune responses?

  Specific and nonspecific immune responses.
• What is a characteristic of nonspecific immune responses?

  Nonspecific responses are the same for all pathogens.
• What is a characteristic of specific immune responses?

  Specific responses involve B and T lymphocytes that target complementary pathogens.
• How do the timing of specific and nonspecific immune responses differ?

  Nonspecific responses are immediate, while specific responses have a time lag.
• What are the two types of specific immune response?

  Cell-mediated and humoral responses.
• What is the first step in the cell-mediated immune response?

  Complementary TH lymphocytes bind to foreign antigens on APCs.
• What do TH lymphocytes release to stimulate other immune cells?


  They release cytokines.
• What is the outcome of clonal expansion of TH cells?

  They become memory cells or trigger the humoral response.
• What do cytotoxic T cells (TC) do during the cell-mediated response?

  They secrete the enzyme perforin to destroy infected cells.
• What is the first step in the humoral immune response?

  Complementary TH lymphocytes bind to foreign antigens on antigen-presenting T cells.
• What happens after TH lymphocytes bind to antigens in the humoral response?

  They release cytokines that stimulate clonal expansion of B lymphocytes.
• What do B cells differentiate into during the humoral response?

  B cells differentiate into plasma cells.
• What do plasma cells secrete?

  Plasma cells secrete antibodies with a complementary variable region to the antigen.
• What are antibodies?


  Proteins secreted by plasma cells.
• What is the structure of an antibody?

  It has two light chains and two heavy chains held together by disulfide bridges.
• What is the function of the variable region of an antibody?

  It has a specific tertiary structure complementary to an antigen.
• What is the constant region of an antibody?

  The rest of the antibody molecule that does not vary between different antibodies.
• How do antibodies lead to the destruction of a pathogen?

  They form an antigen-antibody complex that results in agglutination, enhancing phagocytosis.
• What are monoclonal antibodies?

  Antibodies produced from a single clone of B cells.
• What are memory cells?

  Specialized TH/B cells produced from the primary immune response.
• What is the function of memory cells?

  They can divide rapidly by mitosis if the organism encounters the same pathogen again.
• How do primary and secondary immune responses differ?

  The secondary response has a faster rate of antibody production and a shorter time lag.
• What are the characteristics of the secondary immune response?

  Higher concentration of antibodies and antibody levels remain higher after the response.
• What causes antigen variability?

  Random genetic mutations change the DNA base sequence.
• How does antigen variability affect the structure of antigens?


  It results in a different primary structure of the antigen, affecting its tertiary structure.
• How does antigen variability affect immunity?

  Memory cells may no longer be complementary to the antigen, leading to a lack of immunity.
• What is herd immunity?

  Vaccinating a large proportion of the population reduces available carriers of the pathogen.
• What are some ethical issues surrounding the use of vaccines?

  Production may involve the use of animals and potentially dangerous side effects.
• What is the structure of HIV?

  HIV has genetic material (2 x RNA) and viral enzymes surrounded by a capsid.