Immune system

There are three lines of defense, first, second,and third line of defences.

First line of defence (level 1)

“level 1” Is physical, chemical, and microbiota barriers to prevent or impede entry of pathogens.
they protect the outer and inner layer
they are innate (born with) and non-specific (will act the same with any pathogen, and no memeory retained)
they are highly effective and stop 90% of bacteria/ viruses form entering the body.
physical barriers of the 1st line of defence are tears, mucus, nasal hairs, intact skin.

Second line of defence (level 2)

their aim is to destroy and stop spread of all pathogens inside the body.
they are innate (born with) and non-specific (will act the same way towards any pathogen).
they involve complement proteins, phagocyted eosinophilis, natural killer cells, mast cells and histamines, inflamation and fever.
it involves the action of leukocytes (white blood cells) and proteins.
Phagocytes are ‘cell eaters’, they include neutrophlilis (most prolific cells; first on scene); macrophages and lymphatic system.
Phagocytes are produced in the bone marrow and travel through the circulatory and lympathic system.
Phagocytes purpose is to engulf and digest foreign material incl, bacteria, virsues and dead or injured cells.
Phagocytes have the ability to squeeze through the capillary walls and move into the body tissues in order to engulf foreign materials.
Dendtritic cells are more important in acting as antigen presenting cells, they present the engulfed to the B and T lymphocytes in the third level of defence.
Macrophages are specialised cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms.

  ## Third line of defence (level 3)
  * they rely on the antigens being recognised as foreign, meaning they don’t have MHC I markers.
  * they are a specific response, and their only target/ aim is to destroy identified pathogens/ antigens.
  * it is an adaptive response as it changes over time
  * it involves the action of lymphocytes known as B and T cells that originate from stem cells in the bone marrow.
  * B cells mature and migrate to lymphatic organs, such as nodes.
  * immature T cells migrate to the thymus gland, there they can mature, then mirgate to the lymphatic organs, such as nodes.
  * the humoral immune response is associated with the non-cellular part of the blood and involves the action of antibodies secreted the B cells.
  * the humoral immune system protects the body against extracellular threats e.g. free floating pathogens, toxins/venoms
  * the cell mediated immune response is associated with the production of T cells. It is most effective with intracelluar threats e.g. viral infected cells, DNA damaged cells .
  * cell mediated immune response is responsible for organ donation rejection, it destroys cells by inducing apoptosis by releasing toxins.
  * APC presents a specific antigen to helper t cell in the lymph node
  * Helper t cells release cytokines which are signalling molecules that differentiate the T cell into a cytoxic T cell (killer T cell)
  * cytoxic T cells roam the tissues searching for cells with altered MHC I markers/ antigen fragments
  * the infected cell undergoes apoptosis as the cytoxic cell releases chemicals.

  ### Humoral repsonse
  * the humoral immune system deals with pathogens that are freely circulating outside,to outside the infected cell.
  * B cells are capable of producing large quantities of antibodies called immunoglobulins which target at fight specific pathogens.
  * an undifferentiated B cell has a number of the same specific antibody embedded into its plasma membrane.
  * immature B cells can differentiate into memory cells or plasma cells once sent a signal (cytokine) by a T helper cell.

    ### Antibodies
    * antibodies are proteins
    * they consist of 4 polypeptide chains arranged in a ‘Y’ shape.
    * they are called immunoglobins.
    * there are 2 light chains (shorter) and 2 heavy chains (longer)
    * these two antigen binding sites are identical and are complementary to a particular antigen. (lock and key model)
    * there are a total of 5 different classes of antibodies
    * immunoglobulin G (igG) is the most common type of antibody
    * immunoglobulin E (igE) are involved in the allergic response.
    * once released into the bloodstream and lymph fluid, the antibodies produced by B plasma cells circulate the body.
    * when a specific antigen is located, antigen/antibody complex is formed, resulting in actions that destroy the pathogen.
    * Activated B cells divide rapidly and differentiates B plasma cells and B memory cells
    * Plamsa cells have an exact antibody. With plasma cells, antibodies are released into the blood stream.
    * memory B cells they are long lived, tey don’t secrete antibodies, but display in membrane, ready if pathogen re-invades.

  ### Lymphatic system
  * fluid that leaks from the capillaries and surrounds the tissues is called extracellular fluid. it supplies nutrients and oxygen to the tissues, and removes pathogens and waste.
  * some tissue fluid returns directly into the bloodstream, but some is drained/ filtered through the lymphatic system. this fluid is called lymph and contains only WBC.
  * the lympathic system consists of vessels and lymph nodes.
  * the vessels are separate to the circulatory system but returns fluids and proteins into the blood
  * lymph vessels have valves that prevent backflow of lymph fluid.
  * lymph fluid is filtered by the lymph nodes which swell when infected because lymphocycted produce rapidly to fight of pathogens.
  * lymph nodes are the primary sites where the destruction of pathogens and other substances occur.
  * important components of the lymphatic system are: tonsils, thymus gland, spleen,a nd bone marrow.

  ### Antigen presenting cells (APC)
  * antigen presenting cells are the link between the 2nd and 3rd line of defence
  * they digest and present their MHC II markers, APC are the only ones that can have MHC II markers.
  * they are phagocytic cells from the 2nd line of defence
  * they present the information about the antigen to helper t cells which signal the response.
  * the more non-self antigens an immune system detects, the greater its reaction against a cell it comes into contact with
  * the cell medicated and humoral response are specifically targeting the identified pathogen by searching for the specific antigen that has been presented to them.
  * the antigen can be specifically targeted by humoral and cell mediated responses and immunity (memory) which are created to fight of the pathogen
  * \

  ### Complement proteins
  * they are proteins that are free floating in the blood stream and complement the action of phagicytes and anitbodies.
  * they can react directly with antigens on the surface of the intruders
  * when they are activated the complement proteins can cause opinisation, aggregation, and lysis of cell walls.
  * opinisation: (coating of a particle with proteins) that facilitate and increase phagocytosis of the particle.
  * Aggregation: working with the antibodies to clump pathogen and stop spread/ increase phagocytosis.
  * Lysis of cell walls: (e.g punch a hole) so that cell contents leak out and the cell dies.

    ### Inflamation
  * it occurs when stimualted mast cells release histamine (chemical) via exocytosis.
  * histamine causes vasodilation, increaing blood flow to an area as vessels become more permable.
  * it results in a increased number of leukocytes such as phagocytes, natural killer cells, and complement proteins to destroy the pathogens.
  * this causes swelling and redness in the area.
  * an allergic response is an overeaction of the immune system to relatively harmless substances, because it results in a mass release of histamine.

Involves self MHC I) vs non-self (antigen) recognition:

self marker (MHC) labels the body’s cells as a ‘friend’ and are tolerated by the immune system.
Antigens are molcules that the immune system recognises as foreign (non-self) and treats as a ‘foe’
Immune system (white blood cells) are able to identify self vs non-self through use of MHC i markers and anitgens.
if the self recognition system breaks down, the third line of defence will attack self cells= autoimmune disease.
Antigens are a foreign substance that triggers an immune system response. this includes toxins, venoms, and protein markers on the surfaces of pathogens, non self blood cells, and the cells of transplanted organs.

Leukocytes (WBC)

they arise from pluripotent cells.
pluripotent cells are ‘stem cells’ that can develop into a specifc type of specialised cells.
leukocytes are catergorsied into myeloid stem cells and lymphoid stem cells.
Myeloid stem cells are neutophils, esonophils, basophils. marocphages, dendritic cells.
Lymphoid stem cells include large granular lymphocytes (natural killer cells, NKC), T lymphocytes (cytotoxic cells, memory t cells), and B lymphocytes (plasma cells, and memory cells).
Dendtritic cells are a type of phagocytes and APC and are innate, their main function is to capture, process, and present antigens to adaptive immune cells.

primary response is the response that ocurs the first time an antigen is encountered.

active immunity is natural and artificial, it involves making your own B memory cells through exposure to antigen either naturally such as infection or artificially such as vaccines.

passive immunity is natural and artificial, they do not involve making B memory cells but just gaining temporary immunity though being given antibodies specific to antigens.