The Specific Immune System Flashcards
Antigens
All cells have molecules called antigens on their surfaces. The body recognises the difference between self antigens on your own cells and non-self antigens on the cells of pathogens. Some toxins also act as antigens. Antigens trigger an immune response, which involves the production of polypeptides called antibodies.
The specific immune system (also known as active or acquired immunity) is slower than the non-specific responses-it can take up to 14 days to respond effectively to a pathogen invasion. However, the immune memory cells mean it reacts very quickly to a second invasion by the same pathogen.
Antibodies
Y-shaped glycoproteins called immunoglobulins, which bind to a specific antigen on the pathogen or toxin that has triggered the immune response. There are millions of different antibodies, and there is a specific antibody for each antigen.
How are antibodies made up
Antibodies are made up of two identical long polypeptide chains called the heavy chains and two much shorter identical chains called the light chains. The chains are held together by disulfide bridges and there are also disulfide bridges within the polypeptide chains holding them in shape.
Antibodies bind to antigens with a protein-based ‘lock-and-key’ mechanism similar to the complementarity between the active site of an enzyme and its substrate. The binding site is an area of 110 amino acids on both the heavy and light chains, known as the variable region. It is a different shape on each antibody and gives the antibody its specificity. The rest of the antibody molecule is always the same so it is called the constant region.
When an antibody binds to an antigen it forms an antigen-antibody complex
The hinge region of the antibody provides the molecule with flexibility, allowing it to bind to two separate antigens, one at each of its antigen-binding sites.
How antibodies defend the body
1.The antibody of the antigen-antibody complex acts as an opsonin so the complex is easily engulfed and digested by phagocytes.
2.Most pathogens can no longer effectively invade the host cells once they are part of an antigen-antibody complex.
3.Antibodies act as agglutinins causing pathogens carrying antigens-antibody complexes to clump together. This helps prevent them spreading through the body and makes it easier for phagocytes to engulf a number of pathogens at the same time.
4.Antibodies can act as anti-toxins, binding to the toxins produced by pathogens and making them harmless.
Lymphocytes and the immune response
The specific immune system is based on white blood cells called lymphocytes. B lymphocytes mature in the Bone marrow, while T lymphocytes mature in the Thymus gland.
The main types of T lymphocytes
-T helper cells-these have CD4 receptors on their cell-surface membranes, which bind to the surface antigens on APCs. They produce interleukins, which are a type of cytokine (cell-signalling molecule). The interleukins made by the T helper cells stimulate the activity of B cells, which increases antibody production, stimulates production of other types of T cells and attracts and stimulates macrophages to ingest pathogens with antigen-antibody complexes.
-T killer cells-these destroy the pathogen carrying the antigen. They produce a chemical called perforin, which kills the pathogen by making holes in the cell membrane so it is freely permeable.
-T memory cells-these live for a long time and are part of the immunological memory. If they meet an antigen a second time, they divide rapidly to form a huge number of clones of T killer cells that destroy the pathogen.
-T regulator cells-these cells suppress the immune system, acting to control and regulate it. They stop the immune response once a pathogen has been eliminated, and make sure the body recognises self antigens and does not set up an autoimmune response. Interleukins are important in this control.
The main types of B lymphocytes
-plasma cells-these produce antibodies to a particular antigen and release them into the circulation. An active plasma cell only lives for a few days but produces around 2000 antibodies per second while it is alive and active.
-B effector cells- these divide to form the plasma cell clones
-B memory cells-these live for a very long time and provide the immunological memory. They are programmed to remember a specific antigen and enable the body to make a very rapid response when a pathogen carrying that antigen is encountered again.
Cell-mediated immunity
T lymphocytes respond to the cells of an organism that have been changed in some way, for example by a virus infection, by antigen processing or by mutation (eg cancer cells) and to cells from transplanted tissue. The cell-mediated response is particularly important against viruses and early cancers.
Stages of cell-mediated immunity
- In the non-specific defence system, macrophages engulf and digest pathogens in phagocytosis. They process the antigens from the surface of the pathogen to form antigen-presenting cells (APCs).
- The receptors on some of the T helper cells fit the antigens. These T helper cells become activated and produce interleukins, which stimulate more T cells to divide rapidly by mitosis. They form clones of identical activated T helper cells that all carry the right antigen to bind to a particular pathogen.
- The cloned T cells may:
-develop into T memory cells, which give a rapid response if this pathogen invades the body again.
-produce interleukins that stimulate phagocytosis
-produce interleukins that stimulate B cells to divide.
-stimulate the development of a clone of T killer cells that are specific for the presented antigen and then destroy infected cells.
Humoral immunity
Body responds to antigens found outside the cells like bacteria and fungi, and APCs. The humoral immune system produces antibodies that are soluble in the blood and tissue fluid and are not attached to cells.
B lymphocytes have antibodies on their cell-surface membrane and there are millions of different types of B lymphocytes, each with different antibodies. When a pathogen enters the body it will vary specific antigens, or produce toxins that act as antigens. A B cell with the complementary antibodies will bind to the antigens on the pathogen, or to the free antigens. The B cell engulfs and processes the antigens to become an APC.
Stages of humoral immunity
1.Activated T helper cells bind to the B cell APC. This is clonal selection-the point at which the B cell with the correct antibody to overcome a particular antigen is selected for cloning
2.Interleukins produced by the activated T helper cells activate the B cells.
- The activated B cell divides by mitosis to give clones of plasma cells and B memory cells. This is clonal expansion.
- Cloned plasma cells produce antibodies that fit the antigens on the surface of the pathogen, bind to the antigens and disable them, or act as opsonins or agglutinins. This is the primary immune response and it can take days or even weeks to become fully effective against a particular pathogen. This is why we get ill-the symptoms are the result of the way our body reacts when the pathogens are divided freely, before the primary immune response is fully operational.
5.Some cloned B cells develop into B memory cells. If the body is infected by the same pathogen again, the B memory cells divide rapidly to form plasma cell clones. These produce the right antibody and wipe out the pathogen very quickly, before it can cause the symptoms of disease. This is the secondary immune response.
Autoimmune diseases
Sometimes the immune system stops recognising ‘self’ cells and starts to attack healthy body tissue. This is termed an autoimmune disease. Scientists still do not understand fully why this happens. Immunosuppressant drugs are used to treat the disease.
Three examples of autoimmune diseases
-type 1 diabetes-affects the insulin secreting cells of the pancreas- is treated with insulin injections, pancreas transplants and immunosuppressant drugs
-rheumatoid arthritis- affects the joints, especially in the hands, wrists, ankles and feet.- No cure, helped with anti-inflammatory drugs, steroids, immunosuppressants and pain relief.
-lupus- often affects skin and joints and causes fatigue, can attack any organ in the body including kidneys, liver, lungs or brain- no cure, helped with anti-inflammatory drugs, steroids, immunosuppressants and various others.
