Cell recognition and the immune system

• Describe the replication cycle of HIV

  -HIV attachment protein binds to T cell receptor as it is complementary

  -HIV lipid envelope fuses with the T cell membrane

  -RNA is released into the T cell cytoplasm

  -Reverse transcriptase converts RNA into cDNA

  -cDNA inserts into T cells DNA

  -New RNA is made during transcription

  -New proteins are made during translation

  -New lipid envelope forms from the T cell membrane and the virus is released
• Describe the process of phagocytosis

  -The phagocyte recognises the pathogen through its foreign antigen. The receptors bind to the antigens

  -The phagocyte engulfs the pathogen

  -It encloses the pathogen inside a phagosome

  -Lysosomes fuse with the phagosome and release lysozymes

  -Lysozymes break down the pathogen

  -Phagocyte presents antigens from the pathogen on its cell surface membrane becoming an antigen-presenting cell (APC). The rest of the pathogen is released by exocytosis
• Describe a T lymphocyte

  Produced in the thymus gland

  Activated by APC by the cellular response

  There are two types - helper and cytotoxic
• Describe a B lymphocyte

  Produced in bone marrow

  Activated by free antigens by the humoral response

  There are two types - plasma and memory
• Describe the cellular response

  -The phagocyte presents a foreign antigen on its cell membrane

  -A helper T cell with a receptor complementary to the antigen binds to it

  -Tcell divides by mitosis to form a large clone

  -Some helper T cells release cytokines which stimulate B cells to differentiate and produce antibodies

  -Some cytokines stimulate cytotoxic cells to destroy infected body cells

  -Some cytokines stimulate more phagocytes to engulf pathogens
• Describe the humoral response

  -A B cell binds to a free antigen in the blood with a complementary response

  -A helper T cell binds to antigen on B cell using own complementary receptor

  -Releases cytokines to stimulate B cell to undergo clonal selection and divide by mitosis

  -Some B cells differentiate into plasma B cells which produce and release antibodies complementary to the antigen

  -Other B cells differentiate into memory B cells which circulate in the blood and rapidly divide into plasma B cells if the same antigen returns
• Define an antibody

  A protein that is made by plasma B cells that are complementary to a specific antigen
• Describe the antibody response

  -The variable region binds to two antigens (antibody-antigen complex)

  -This causes agglutination of pathogens

  -Phagocytes attract to constant regions and they can engulf and destroy at a faster rate
• Describe herd immunity

  When a large number population have memory B cells so the people without are less likely to become infected as there is less transmission
• Describe antigenic variability

  The ability of pathogens to change their surface antigens through mutations in DNA/RNA which code for them

  This means disease is more likely to occur and vaccines are less likely to work as memory B cells and antibodies will no longer recognise these antigens
• Describe active immunity

  -Provides long term protection

  -Involves memory B cells

  -Protection takes a while to develop

  -Produces antibodies using plasma B cells after exposure to a pathogen
• Describe passive immunity

  -Provides short term protection

  -Does not involve memory B cells

  -Provides immediate protection

  -Antibodies are transferred from another individual or artificial source
• Describe how AIDS weakens immunity and increases susceptibility to opportunistic infections

  -Fewer helper T cells as they are destroyed by HIV

  -So fewer B cells are activated so fewer antibodies are produced against foreign antigens

  -And fewer cytotoxic T cells are activated so fewer infected body cells are destroyed
• What cellular processes do antibodies inhibit?

  -Cell wall production

  -Protein synthesis

  -DNA replication
• What is the role of monoclonal antibodies in pregnancy tests?

  They are used to detect the presence of hCG in urine.
• What happens when urine is applied to the test pad in a pregnancy test?

  If the person is pregnant, the urine will contain hCG which binds to mobile monoclonal antibodies.
• What is attached to the mobile monoclonal antibodies in the pregnancy test?

  An enzyme is attached to the mobile monoclonal antibodies.
• What occurs when hCG is present in the pregnancy test?

  The hCG binds to the mobile monoclonal antibodies, forming antibody-hCG complexes.
• What happens to the antibody-hCG complexes in the pregnancy test?

  They diffuse to a test line containing immobilised monoclonal antibodies that bind to hCG.
• What is the result of the enzyme reacting with a substrate at the test line?

  A color change occurs, indicating a positive result for pregnancy.
• What is the purpose of the control line in a pregnancy test?

  It confirms that the test is working correctly by showing a color change regardless of hCG presence.
• What happens to some mobile antibodies in the pregnancy test?

  They diffuse further to a control line and bind to monoclonal antibodies there.
• What is the outcome when the enzyme reacts with a substrate at the control line?

  A color change occurs, confirming the test's validity.
• Describe the ELISA test for antigens

  -A primary antibody is attached to the bottom of the well

  -A sample is added and the antigen binds to the antibody

  -The well is washed to remove unbound antigen

  -A second antibody is added with an enzyme attached which binds to the antigen

  -The well is washed to remove unbound antibody

  -The substrate for the enzyme is added and a colour change occurs
• Describe the ELISA test for antibodies

  -The antigen is attached to the bottom of the well

  -The sample is added and the antibody binds to the antigen

  -The well is washed to remove unbound antibody

  -A second antibody is added with an enzyme attached which binds to the first antibody

  -The well is washed to remove unbound antibody

  -The substrate for the enzyme is added and a colour change occurs