4.1.7(Antibodies)

Describe what antibodies are and their role

• They are proteins or glycoproteins found on the surface of cells that stimulate an immune response

• They are specific to each antigen and are produced when foreign antigens are detected by the immune system

• Their is to bind onto antigens to form an antigen-antibody complex so the phagocyte or lymphocyte is then able to engulf it

State what antigens are also known as

• they are known as immunoglobulins

Describe the structure of antibodies

• They are a Y-shaped molecule that consists of 4 polypeptide chains

  • 2 identical light polypeptide chains

  • 2 Identical heavy polypeptide chains

• The polypeptide chains are held by disulphide bridges

• Each polypeptide chain is a tertiary structure that consists of distinct domains - variable region and constant region

• They have a hinge region that contains disulphide bridges which gives flexibility so the molecule is able to bind with more than one antigen

Describe what distinct and variable regions are

• Variable regions are parts on antibodies which have a shape specific to the shape of the antigen

• Constant regions are parts on antibodies which are the same in all antibodies- it may have a site for easy binding of phagocytic cells

Describe and explain how antibodies are specific

• The variable region of the polypeptide chains have a different amino acid sequence for every antibody

• This creates a different tertiary structure for the binding site on each antibody

• This means that only one antibody will be able to bind to a specific antigen

• Each antibody is complementary to each antigen

State the 3 types of antibody

• Opsonins

• Agglutinins

• Anti-toxins

State where opsonins and phagocytes bind to pathogens

• Opsonins bind to the pathogens at the variable region

• Phagocytes bind to pathogens at the constant region

Describe and explain the role of opsonins

• opsonins are protein markers that bind to the antigens on the plasma membrane on a pathogen once detected as a foreign antigen and aid phagocytosis.

• They do this by acting as binding sites for phagocytic cells, so that these phagocytes can more easily bind and kill the pathogen

Describe the process of neutralisation using opsonins

• Pathogens may use their antigen molecules as a binding site used for attachment to a host cell

• The opsonin in this case binds to the specific antigen and stops the antigen from binding to the host cell

• This stops pathogens from entering a host cell

Describe and explain the role of agglutinins

• Each antibody can bind to 2 different antigens at the same time as theyhave 2 identical binding sites, they are able to use this to for crosslinks by bindning an antigen of one pathogen with one binding site and then an antigen on another pathogen with its other binding site

• When this process is repeated many times, crosslinks clump pathogens together which is called Agglutination

State the 2 advantages of agglutination

• The agglutinated pathogens are immobilised so they can no longer enter host cells

• Agglutinated pathogens are readily and easily engulfed by phagocytes

Describe and explain the role of antitoxins

• They bind to toxic molecules which are released by pathogens

• Antitoxins undergo neutralisation as well and bind to toxins which stops toxins from binding to pathogens

Describe and explain what happens during primary and secondary responses

• Primary response:

  • The primary response is the first response when the pathogen enters the host for the first time

  • Its slow and it takes a few takes for the number of antibodies in the blood to rise to a level that can combat the infection successfully, this is due to the lack of B cells that make the specific antibody needed to bind to foreign antigen

  • Once the pathogen has been removed, the number of antibodies drops quickly but B memory cells stay in the body

• Secondary response:

  • The secondary response is anytime after the first time a new pathogen has entered the host

  • Its much quicker and much more effective. This is due to B and T memory cells that remained in the blood after the primary response:

    • B memory cell: Quickly divide to form plasma cells which secrete the correct Ab to the antigen

    • T memory cells: Quickly divide into the right type of T-cells to kill cells carrying the pathogen

  • These can recognise the foreign antigen and can stimulate a much quicker immune response

  • This causes a rapid rise in concentration of specific antibodies in the blood