The immune response

• A phagocyte (e.g. a macrophage)is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They're found in the blood and in tissues and are the first cells to respond to an immune system trigger inside the body
• 1. A phagocyte recognises the foreign antigens on a pathogen
  2. the cytoplasm of the phagocyte moves around the pathogen, engulfing it
  3. the pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte
  4. a lysosome (an organelle that contains enzymes called lysozymes) fuses with the phagocytic vacuole, the lysozymes break down the pathogen
  5. the phagocyte then presents the pathogen's antigens - it sticks the antigens on its surface to activate other immune system cells. The phagocyte is acting as an antigen-presenting cell
• T-cells: A T-cell (T-lymphocyte) is another type of white blood cell. It has receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes - this activates the T-cell
• Different types of T-cells respond in different ways e.g. helper T-cells (TH cells) release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells (TC cells), which kill abnormal and foreign cells. TH cells also activate B-cells, which secrete antibodies
• B-cells: also called B-lymphocytes are also a type of white blood cell. They are covered with antibodies - proteins that bind to antigens to form an antigen-antibody complex
• Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens
• When a B-cell encounters an antigen, the antibody on its membrane binds to the antigen. This causes the B-cell to divide rapidly into many identical daughter cells, all of which have the same antibody on their membranes.
• The new B-cells produced from the original one are called clones because they all have the same characteristics
• Some of these cloned B-cells become plasma cells, which produce large amounts of antibodies specific to the antigen that activated them
• The new B-cells produced from the original B-cell are plasma cells. Plasma cells produce large amounts of the specific antibody they carry on their membranes. These antibodies circulate through the body until they encounter the antigen again. When they do, they bind to the antigen forming an antigen-antibody complex
• Plasma cells can live for several months or even years, producing more antibodies as needed
• Some of these cloned B-cells become plasma cells, which produce large quantities of the specific antibody that was originally present on the parent B-cell's membrane
• When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it. This together with substances released from helper T-cells, activated the B-cell. This process is called clonal selection.
• The activated B-cell divides into plasma cells
• Antibody production: plasma cells are identical to the B-cell (they're clones). The secrete loads of antibodies specific to the antigen - monoclonal antibodies
• Monoclonal antibodies bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes
• An antibody has 2 binding sites, so can bind to 2 pathogens at the same time. This means that pathogens become clumped together - this is called agglutination. Phagocytes than bind to the antibodies and phagocytose many pathogens at once. This process leads to the destruction of pathogens carrying this antigen in the body
• Antibodies are proteins - they're made up of chains amino acids. The specificity of an antibody depends o its variable regions, which form the antigen binding sites.
• Each antibody has a variable regions with a unique tertiary structure (due to different amino acid sequences) that's complementary to one specific antigen - all antibodies have the same constant regions
• The immune response is split into 2 - the cellular response and the humoral response
• Cellular - the T-cells and other immune system cells that they interact with e.g. phagocytes, form the cellular response
• Humoral - B-cells, clonal selection and the production of monoclonal antibodies form the humoral response
• The primary response: when an antigen enters the body for the first time, it activates the immune system - the primary response. It is slow because there aren't many B-cells 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 show symptoms of the disease
• After being exposed to the antigen, both T and B-cells produce memory cells. These memory cells remain in the body for a long time.
• Memory T-cells remember the specific antigen and will recognise it a second time round
• Memory B-cells record the specific antibodies needed to bind to the antigen
• The person is now immune - their immune system has the ability to respond to a second infection.
• The secondary response: If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response - the secondary response
• Clonal selection happens faster. Memory B-cells are activated 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 carry the antigen.
• The secondary response often gets rid of the pathogen before the person begins to show any symptoms