cells and the immune system

• What is an antigen?

  A foreign protein that stimulates an immune response.  Found on the cell surface membrane of cells and found on pathogens. 
• What is antigenic variability?

  Some pathogens exhibit antigen variability. Their antigens change frequently due to genetic mutations.

  The surface receptors on lymphocytes and memory cells are complementary in shape to only one antigen, so when the antigen on a pathogen changes the lymphocytes and memory cells can no longer bind.

  As a result, there is no secondary immune response and the host gets infected and suffers from the disease again.
• What is the process of phagocytosis?

  1. Phagocytes are attracted to chemicals released by pathogens and damaged body cells (chemotaxis). 
  2. They recognise foreign antigens, bind to the pathogen, and engulf it via endocytosis, forming a phagocytic vacuole (phagosome). 
  3. A lysosome fuses with the phagosome, forming a phagolysosome, and releases lysozymes to digest the pathogen. 
  4. The phagocyte then presents the pathogen’s antigens on its cell membrane/surface, acting as an antigen-presenting cell to activate the immune response.
• What is a phagocyte?

  Phagocyte (e.g. a macrophage) is a type of white blood cell that carries out phagocytosis. They are the first cells to respond to an immune system trigger inside the body.
• What are T-cells?

  • T-cells are activated when they bind to a specific antigen that is being presented by one of the host’s cells (APC or body cell invaded by pathogen)
  • T-cells (with receptors specific to the antigen) divide by mitosis to increase in number 

  T-lymphocytes can differentiate into two main types of T cell:

  • helper T cells
  • cytotoxic T cells(also known as killer T cells)
• What do helper T-cells do?

  Helper T cells stimulate:

  • The maturation of B-cells into plasma cells
  • The production of memory B cells
  • The activation of cytotoxic/killer T cells
• What do cytotoxic/killer T cells do?

  • T killer cells patrol body in search of antigen presenting body cells
  • T killer cells attach to the foreign antigens on the cell surface membranes of infected cells and punch a hole in the cell surface membrane, allowing toxins to enter that kill the infected body cells, along with the pathogen inside
• What do B-cells do?

  • Once mature, each type of B-cell can make one type of antibody molecule
  • Each antibody molecule can combine specifically with one type of antigen
  • If that antigen enters the body, B-cells with the correct cell surface receptors will be able to recognise it and bind to it (clonal selection)

  These specific B-cells divide repeatedly by mitosis (clonal expansion) and differentiate into plasma cells or memory cells.
• What happens when an antigen first enters the body? (primary immune response)

  When an antigen enters the body for the first time, macrophage presents antigen on its surface, T cell with complementary receptors bind to antigen and stimulates B cells. B cells bind to specific antigen. The small amount of B-cells with receptors complementary to that antigen divide by mitosis stimulated by cytokines.
• What do B-cells do after division? (primary immune response)

  • Some of the B-cells become plasma cells that secrete lots of specific antibody molecules into the blood
  • The antibodies they secreted stay in the blood for a longer time
  • Some of the B-cells become memory cells that remain circulating in the blood for a long time
  • The memory cells remain in the body for a long time and remember the specific antigen so they produce plasma B cells and antibodies faster.
  • The person is now immune — their immune system has the ability to respond quickly to a second infection. 
• What happens in the secondary response?

  • This response is very quick
  • Memory B-cells are activated as they recognise the pathogen and divide into plasma cells that produce the right antibody to the antigen. 
  • Memory T-cells are activated and divide into the correct type of T-cells to kill the cell carrying the antigen.
  • The antibody concentration in the blood takes a much shorter period of time to increase and is higher than after the vaccination or first infection
• What are antibodies?

  Antibodies are proteins specific to an antigen, produced by B-cells. 
• What is the structure of antibodies

  Antibodies have a quaternary structure , with two heavy polypeptide chains and two light polypeptide chains. Disulfide bridges joins the two different polypeptides.

  • Each polypeptide chain has a constant/variable region - the variable region has a specific amino acid sequence and is where the antibody attaches
  • At the end of the variable region is a site called the antigen-binding site. The shape of binding site is complementary to specific antigens, forming an antigen-antibody complex
• What is the antigen-antibody complex?

  An antigen and its complementary antibody have complementary molecular shapes. 

  When an antibody collides with a foreign cell, it binds with one of the antigens. 

  When this occurs, the two molecules combine to form an antigen-antibody complex.
• What processes can happen once a antigen-antibody complex is formed?

  • Antibodies have at least two antigen-binding sites meaning they can bind to more than one pathogen at a time causing groups of the same pathogens to become clumped together - This process is known as agglutination
  • The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogens
  • Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together