Sometimes the body comes under attack from pathogens (microorganisms that can cause disease). If pathogens get into our body, it is down to our immune system to spring into action and destroy them. In this article we will be looking at how foreign antigens trigger our immune response, and how that results in the pathogens being destroyed.
Firstly, what is a foreign antigen? Antigens are molecules, normally proteins or polysaccharides, which are found on the surface of all cells. The body recognises the antigens found on its own cells and does not trigger an immune response to them (unless there is an autoimmune condition). Any other antigens that have made their way into the body are foreign, and if the immune system detects these, it will initiate the immune response. They could be from:
- Abnormal body cells e.g. cancerous or infected cells.
- Cells from another individual of the same species e.g. from an organ transplant.
Non-specific, specific, cellular, or humoral immune response?
Depending on which A-Level biology specification you are learning, you may have come across some or all of these terms.
- The non-specific immune response provides rapid protection and responds in the same way to all invading pathogens. It includes physical barriers (covered in a future article), inflammation, and phagocytosis.
- The specific immune response is activated more slowly and targets specific antigens. It is also responsible for producing memory cells for future immunity. The specific immune response can be broken down into the cellular and humoral response.
- The cellular response involves activation of T-cells by antigen-presenting cells, and the action of T-cells.
- The humoral response involves activation and action of B-cells to produce monoclonal antibodies specific to the foreign antigen. The antibodies can then help to destroy the pathogen.
See the summary diagram at the bottom of the page to see how all these responses link together. Now, let’s have a closer look at the processes and cells involved in the immune response.
Phagocytes are a type of white blood cell. Different types of phagocytes include neutrophils (which have a multi-lobed nucleus) and macrophages. Their job is to carry out phagocytosis:
- The foreign antigens are detected by the phagocyte, and the cytoplasm surrounds the pathogen; the pathogen is engulfed.
- The pathogen is contained in a phagocytic vacuole. The vacuole fuses with a lysosome, and the digestive enzymes (lysozymes) are released from the lysosome. The pathogen is broken down.
- The phagocyte presents the pathogen’s antigens on it’s surface, and can now be called an antigen-presenting cell.
Activation and action of T-cells
T-cells (T-lymphocytes) are another type of white blood cell involved in the immune response. The nucleus takes up most of the space inside the cell, and it has receptors on it’s surface. These receptors binds to a specific complementary antigen, so there are many different T-cells each with a different receptor. If the complementary antigen (presented by an antigen-presenting cell) binds to the receptor, the T-cell becomes activated by clonal selection (i.e. the specific T-cell which needs to be cloned is selected). The next process is clonal expansion – the T-cell divides by mitosis and differentiates into the following:
- T killer cells (or cytotoxic T-cells): kill infected cells by attaching to the antigens.
- T helper cells: release signalling molecules called cytokines which activate T killer cells, B-cells, and phagocytes.
- T regulatory cells: suppress other white blood cells to prevent an autoimmune response.
- T memory cells: remain in the blood ready for when the pathogen is encountered again (see future article).
Activation and action of B-cells
B-cells (B-lymphocytes) look quite similar to T-cells, but have antibodies on their surface rather than receptors. The antibodies are specific to a single antigen and can bind to form an antigen-antibody complex. We will look at the structure of antibodies in another article.
When the complementary antigen binds to the antibodies, the specific B-cell becomes activated (clonal selection). The cytokines released from the T helper cells are also needed for activation. The B-cells divides by mitosis and differentiates into:
- Plasma cells (B effector cells): produce and secrete monoclonal antibodies which are complementary to the foreign antigen.
- B memory cells: remain in the blood for future immunity (see future article).
Here is a diagram to show how all the different parts of the immune response interact. Don’t forget the important terms ‘clonal selection’ and ‘clonal expansion’ as well.