The Immune System

• Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells.
• When a Pathogen (like a bacterium) invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system.
• the immune response involves specific and non-specific stages. The non-specific responce happens in the same way for all microorganisms, whatever foreign antigens they have. The specific response is antigen specific - it is aimed at specific pathogens. it involves white blood cells known as T and B lymphocytes.

1) Phagocytes Engluf Pathogens

2) Phagocytes Activates T lyphocytes 

3) T lyphocytes activates B lymphocytes, which divides into plasma cells 

4) Plasma Cells make more Antibodies to a specific antigen.

a phagocyte is a type of white blood cell that carries out Phagocytosis (engulfment of Pathogens). They are found in the blood and in tissues and carry out a non-specific immune response.

• A phagocyte recognises the antigens on the pathogen.
• The cytoplasm on the Phagocytemoves round the pathogen, engulfing it. this may be made easier by the presence of optosins - molecules in the blood that attach to foreigens to aid phagocutosis.
• the pathogen is now contained in a phagosome (a type of vesicle) in the cytoplasm of the phagocyte.
• A lysosome (an organelle that contains digestive enzymes) fuses with the phagosome. the enzymes breaks down the pathogen.
• The phagocyte the presents the pathogens antigens. It sticks the antigens on its surface to activate other immune system cells. When a phagocyte does this it is acting as an antigen-presenting cell (APC).

Neutrophils are a type of phagocyte. They're the first white blood cells to respond to a pathogen inside the body. Neutrophils move towards a wound in response to signals from Cytokins (proteins that act as messenger molecules). The cytokines are realeased by cells at the site of the wound.

• a T-lyphocyte is another type of white blood cell.
• Their surface is covered with receptors.
• The receptors bind to bind to antigens presented by APCs. 
• Each T-Lymphocyte has a different receptor on its surface.
• When the receptors on the surface of a t-lyphocyte meets a complimentary antigen, it binds to it - so each T-lymphocyte will bind to a different antigen.
• This activates the T-Lyphocyte - the process is called clonal selection.
• The T-Lymphocyte then undergoes clonal expansion - it divides to produce clones of itself. Different types of T-Lympocytes carry out different functions.

Different types of activated T-Lymphocytes include:

• T-Helper Cells: these release substances to activate B Lymphocytes and T-Killer cells
• T-Killer cells: these attach to and kill cells that are infected with a virus
• T-Regulatory cells: these supress the immune response from other white blood cells. This helps to stop immune system cells from mistakenly attacking the hosts body cells

• B-Lymphocytes are another type of white blood cell.
• Theyre covered with proteins called antibodies.
• antibodies bind to antigens to form an antigen-antibody complex.
• Each B-Lymphocyte has a different shapped antibody on its surface.
• When the antibody on the surface of a B-Lyphocyte meets a complimentary shaped antigen, it binds to it - so each B-Lymphocyte will bind to a different antigen.
• This, together with substances released from the T-Helper cells, activates the B-Lymphocyte. This process is another example of clonal selection.
• The activated B-Lymphocyte divides, by mitosis, into plasma cellsand memory cells. this is another example of clonal expansion.

• Cell Singnalling is how cells communicate.
• A cell may release (or present) a substance that binds to the receptors on another cell - this causes a responce of some kind in the other cell.
• cell signalling is really important in the immune response because it helps to activate all the different types of white blood cells that are needed.
• For example, T Helper cells release interleukins (a type of cytokine) that binds to receptors on B-Lymphocytes. This activates the B-Lymphocytes - the Helper T Cells are signalling to the B-Lymphocytes that theres a pathogen in the body

• Plasma cells are clones of the B-Lymphocyte (they're identical to the B-Lymphocytes).
• They secrete loads of the antibody, specific to the antigen, into the blood.
• These antibodies will bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes

• The variable regions of the antibody form the antigen binding sites.
• the shape of the variablle region is complimentary to a patricular antigen. The variable regions differ between antibodies.
• The hinge regions allows flexibility when the antibody binds to the antigen.
• The constant regions allow binding to receptors on immune system cells, e.g. phagocytes. The constant region is the same (i.e. it has the same sequence of amino acids) in all antibodies.
• Disulfide bridges (a type of bond) holds the polypeptide chains of the protein together.

• Agglutinating Pathogens - each antibody has 2 binding sites, so antibodies can bind to two pathogens at a time - the pathogens become clumped together. Phagocytes then bind to the antibodies and phagocytose a lot of pathogens all at once. Antibodies that behave in this way are known as agglutinins.
• Neutralising Toxins - like antigens, toxins have different shapes. Antibodies called anti-toxins can bind to the toxins produced by pathogens. this prevents the toxins from from affecting the human cells, so the toxins are neutralised (inactivated). The toxin-antibody complexes are also phagocytosed.
• Preventing the pathogen binding to human cells -  when antibodies bind to the antigens on pathogens, they may block the cell surface receptors that the pathogens need to bind to the host cells. this means the pathogen cant attach or infect the host cells.

• When a pathogen enters the body for the first time, the antigens on its surface activate the immune system. this is called the primary response.
• The primary response is slow because there arent many B-Lymphocytes that can make the antibody needed to bind to it.
• Eventually the body will produce enough of the right antibody  to overcome the infection. Meanwhile , the infected person will have symptoms of the disease.
• After being exposed to an antigen, both the T and B Lymphocytes produce memory cells. these memory cells remain the body for a long time.
• Memory T-Lymphocytes remember the specific antigen and will recognise it a secound time round. Memory B-Lymphocytes record the specific antibody needed to bind to the antigen.
• The person is now immune - their immune system has the ability to respond quickly to a second infection.

• if the same pathogen enters the body again, the immune system will be quicker, and faster with the immune response - known as the secondary response.
• Clonal selection happens faster. Memory B-Lymphocytes are activated and divide into plasma cells that produce the right antibody to the antigen. Memeory T-Lymphocytes are activated and divide into the correct type of T-Lymphocytes to kill the cell carrying the antigen.
• The secondary responce often gets rid of the pathogen before you get any of the symptoms.