Immunity

• Antigens and Phagocytosis:

  Self vs Non-Self:

  On the surface of all cells are chemical markers (e.g proteins) called antigens. Each antigen has its own unique shape. The more closely related 2 individuals are, the more antigens they have in common. Your body recognises the antigens on your cells as your own (self); anything with diff antigens to you (non-self) stimulates an immune response. In an immune response, your body will recognise the antigen as foreign (therefore bad) and will attack it
• Antigens:

  All cells have specific proteins on their surface membrane that identify it called antigens. An antigen is a protein found on the outside of cells, that trigger an immune response
• Antigens enable the immune system to identify:

  -pathogens (microorganisms that cause disease)

  -cells from other organisms of the same species (e.g in transplanted organs)

  -abnormal body cells (e.g cancerous cells)

  -toxins (poisonous molecules often released by bacteria)
• Phagocytes and White Blood Cells:

  Phagocytes are made in the bone marrow, travel in capillaries but can squeeze through walls into tissues. They both patrol the body, searching for invaders (non-self antigens). There are two types:

  neutrophils and macrophages
• Neutrophils- engulf and digest pathogens (and dead human cells/debris)
• Macrophages- can punch holes in the bacteria or stick proteins to the outside of the bacteria to make them more appealing for the neutrophils to destroy
• Phagocytosis- cellular process of engulfing solid particles using the cell membrane (carried out by phagocytes)
• Phagocytosis:

  1.Pathogen recognised as having foreign/non-self antigens. Pathogen attaches to the phagocyte by surface receptors

  2.The pathogen us engulfed by the phagocyte by endocytosis forming a phagosome
• Phagocytosis:

  3.Lysosomes fuse to phagolysosome and release H2O2 ,HCl and digestive enzymes into phagosome to digest pathogen.

  4.Harmless products removed (egested/excreted) by exocytosis or used by phagocyte
• Cell Mediated Immune Response:

  Phagocytosis is a non-specific response and it is sometimes not enough to cope with large numbers of pathogens e.g the flu virus
• Cell Mediated Immune Response:

  1.Phagocytes which engulf the virus, present some of the pathogen’s antigens on their own cell membrane- they become antigen presenting cells (APCs). This activates T helper cells, which stimulates and recruits more phagocytes and T cells to assist in the immune response. This includes cytotoxic (killer) T cells which destroy infected or foreign cells by releasing chemicals e.g perforin (makes pores in the membrane) into the invaded cell
• Cell Mediated Immune Response:

  2. Activated T cells can then also activate B cells causing them to divide by mitosis, this is the start of the humoral response
• Specific Immune Response:

  Two Parts:

  1.Cell Mediated Response- involves specialised white blood cells called T-lymphocytes which target pathogens inside and outside cells
• Specific Immune Response:

  Two Parts:

  2. Humoral Response- involves specialised white blood cells called B-lymphocytes which target phagocytes by producing antibodies
• Cell-Mediated Immune Response:

  1.APCs can be either phagocytes or cells that have been invaded by the pathogen

  2.Once phagocytosis has taken place, pathogenic antigens are presented on the cell surface membrane
• Cell-Mediated Immune Response:

  3.T cells respond to the cell which is presenting the antigens

  4.T helper cells respond first- they have receptors which fit exactly to the presented antigens
• Cell-Mediated Immune Response:

  5.This activates the T cells and they begin to differentiate and recruit other T cells and activate B cells
• Phagocytes- engulf pathogens and destroy them using lysosomes

  Then become APCs
• APC- present pathogenic antigens on its own cell membrane signalling T lymphocytes
• T Helper- stimulates lots of cells:

  Cytotoxic T cells

  B cells

  Phagocytosis

  Memory T cells
• Cytotoxic T Cells- kill infected cells through the release of an enzyme that kills the cell
• B Cells and Humoral Response:

  1.The B cells get the antigen from pathogens or APCs and presents the antigen on its cell surface membrane. T helper cells bond to this antigen to activate the B cells. The B cells begin to divide by mitosis (clone themselves) to form plasma cells and memory cells this is known as clonal selection. Plasma cells produce antibodies that are complimentary to the antigens (primary response)
• B Cells and Humoral Response:

  2.The memory cells stay in the blood stream circulating. If they come into contact with the antigen again they divide to form plasma cells and memory cells again (secondary immune response)
• Primary vs Secondary Immune Response:

  In the primary response there are not many initial B cells to make the antibody and the antibodies only survive in the bloodstream for a few days.
• Primary vs Secondary Immune Response:

  In the secondary response they produce antibodies much more quickly and more new memory cells enter the bloodstream to provide long-term immunity from reinfection of the pathogen
• Antibodies are proteins which bind to antigens to form an antigen-antibody complex
• Antibody Structure:

  1.Antibodies are proteins made up of four polypeptide chains (quaternary structure),2 heavy and 2 light. Each chain has a variable region and a constant region. The chains are connected by disulphide bridges.
• Antibody Structure:

  2.All antibodies have the same constant regions that allow them to bind to receptors on immune system cells. They have two variable regions which are the antigen binding sites so they will have a unique tertiary structure complementary to an antigen. One antibody can bind to two pathogens at once
• How Antibodies Work: Agglutination- Antibodies bind to antigens on pathogens and cause them to clump together, these clumps are then destroyed by phagocytes. This makes it easier for phagocytes to engulf more pathogens at a time.
• How Antibodies Work:

  Neutralising Toxins- Some antibodies work by neutralising toxins released by pathogens
• How Antibodies Work:

  Preventing Viruses From Entering Host Cells- Viruses have spike proteins on their surface which recognise and bind to receptors on the surface of the host cell which allow them entry into the cell. Antibodies can bind to these on viruses and stop them attaching to their host cells
• Antigenic Variability:

  1.Memory cell function explains why we generally only catch chicken pox once. The pathogens causing each of these diseases are of a single type and thus have the same antigen, they are rapidly identified by the memory cells and a secondary immune response is triggered which quickly wipes out the virus.
• Antigenic Variability:

  2.Some pathogens can form different strains (caused by mutations in genetic material) which often have different antigens- this is called antigenic variation. This means that if you are re-infected, the memory cells will not recognise the antigen and won’t be activated and the antibodies made last time won’t be complimentary so the body has to start again with the primary response so you still get symptoms
• Antigenic Variability:

  3.Antigenic variation is common in influenza and HIV, they make it hard to produce vaccines against these pathogens. Every year a new flu vaccine has to be made that is most effective against the most recently circulating strain of the virus in the population
• Vaccines:

  1.Vaccines are a way of introducing antigens of a dead or weakened pathogen into the body to stimulate the production of antibodies and memory cells. One dose induces a primary response, multiple doses can increase the number of antibodies and memory cells in the blood stream through secondary response.
• Vaccines:

  2.Vaccines are long lasting because they produce memory cells which can produce complimentary antibodies to the antigen. Many vaccines can contain multiple antigens to protect against different strains of pathogens (flu jab) or different diseases (e.g MMR)
• Pathogens prepared for vaccines are made harmless by:

  -Killing but leaving antigens unaffected e.g Cholera vaccine

  -Weakening (attenuation-heating) but leaving antigens unaffected e.g oral vaccine against polio

  -Purified antigens removed from pathogen e.g vaccine against hepatitis B

  -Using inactivated toxins called toxoids that are harmless but trigger same immune response e.g tetanus injection
• Because vaccines contain foreign antigens there will still be a primary immune response which can have symptoms such as swelling, fever, runny nose
• Ethical Issues With Vaccines:

  -All vaccines are tested on animals first (same as all drugs)

  -Humans in clinical trials may put themselves at risk because they believe they may be “immune”
• Ethical Issues With Vaccines:

  -Some people refuse vaccines over fears of side effects, they are protected by herd immunity in the same way that people who can’t get the vaccine

  -If there was a new disease, difficult decisions would be made about who would be first to receive it