(2) Antigens

• the initial response of the body to an infection is non-specific such as phagocytosis
• the specific immune response reacts to specific antigens
• specific response is slow in action at first but can provide long term immunity
• immunity is the ability of organisms to resist infection by protecting against disease causing microorganisms or their toxins that invade their bodies
• immunity involves the recognition of foreign material
• immunity is the ability of the body to resist or fight off disease
• the specific immune response depends on a type of white blood cell called lymphocytes
• lymphocytes are smaller than phagocytes and have a large nucleus that fills most of the cell
• the two types of lymphocytes are :

  • B lymphoctes
  • T lymphocytes
• To defend the body from invasion by foreign material, lymphocytes must be able to distinguish between :

  • the body's own cells and molecules (self)
  • foreign material (non-self)
• the ability to recognise foreign cells is made possible by antigens
• every cell in the human body has markers that identify it - allow cell-to-cell recognition
• Antigens = are specific molecules found on the surface of cells (and viral particles) that can generate an immune response when detected by the body
• antigens are usually proteins
• antigens are often proteins that are part of the phospholipid bilayer (such as glycoproteins - glycolipids)
• proteins (antigens) have enormous variety and a highly specific tertairy structure - this variety of specific 3D structure is what distinguishes one cell from another
• the presence of an antigen triggers the production of an antibody as part of the body's defence system
• an antigen is a specific molecule that can generate an immune response
• Antigens allow the immune system to identify :

  • pathogens
  • no self material - cells from other organisms of the same species
  • toxins
  • abnormal body cells - cancer cells or pathogens infected cells
• example of non-self material :

  • organ transplant
  • blood transfusion
• some pathogens exhibit antigen variability
• antigen variability = the antigens present on pathogens surface change frequently due to genetic mutations
• genetic mutations example :

  • cold and flu virus mutates rapidly, resulting in new strains each year
• specific lymphocytes are not produced in response to an infection but already exist
• as there are lots of different lymphocytes, there are onl a very few of each type
• Lymphocyte action to infection :

  • when an infection occurs pathogen travels in blood
  • specific lymphocyte collides pathogens antigen
  • complementary protein to those of the pathogen is stimulated to divide to build up its number to a level where it can be effective in destroying it - clonal selection
• clonal selection explain why there is a time lag between exposure to the pathogen and the body’s defences controlling it
• clonal selection = mitosis
• Recognising cells belonging to the body :

  • in the foetus lymphocytes constantly collide with other cells - mainly the body's own material (self)
  • some lymphocytes have receptors that fit the body's own cells
  • these lymphocytes either die or are suppressed
  • only remaining lymphocytes are those that might fit foreign material (non-self)
  • so only respond to foreign material
• lymphocytes self material in the foetus
• Recognising slf material in adults :

  • Lymphocytes produced in the bone marrow initially only encounter self-antigens
  • lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes
  • no clones of anti-self lymphocytes will appear in the blood - leaves only those that might respond to non-self antigens
• no clones of anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens
• apoptosis = programmed cell death