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 glycoproteins, which are found on the surface of all cells. The body recognises the antigens found on its own cells and they do not trigger an immune response (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. Foreign antigens could be present on:
- Pathogens
- Toxins
- Abnormal body cells e.g. cancerous or virus-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 studying, 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 divided 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 complementary to the foreign antigen. The antibodies can then assist with destruction of pathogens (read more here).
Now, let’s have a closer look at the processes and cells involved in the immune response.
Phagocytosis
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 phagosome. The phagosome fuses with a lysosome, and the digestive enzymes (lysozymes) are released from the lysosome. The pathogen is digested and destroyed.
- 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 they has receptors on their cell-surface membrane. These receptors binds to a specific complementary antigen found on antigen-presenting cells. The first type of T-cell to be involved are T helper cells. If the specific complementary antigen (presented by an antigen-presenting cell) binds to the receptor, the specific T helper cell becomes activated. It divides by mitosis and the clones differentiate into the following:
- Active T helper cells: release signalling molecules called cytokines which activate cytotoxic T cells (also called T killer cells), activate phagocytes, and stimulate clonal selection of B-cells (see below).
- T memory cells: remain in the body ready for the secondary immune response (see this article).
Cytotoxic T cells (T killer cells) kill virus-infected cells and tumour cells. They do not directly kill pathogens – watch out for this common misconception.
Activation and action of B-cells
B-cells (B-lymphocytes) look quite similar to T-cells, but have antibodies on their cell-surface membrane rather than receptors. The antibodies are complementary to a specific 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 on the pathogen binds to the antibodies on a B-cell, the specific B-cell becomes activated (this is called clonal selection). The cytokines released from the active T helper cells are also needed for activation. The B-cells divides by mitosis (this is called clonal expansion) and the clones differentiate into:
- Plasma cells: produce and secrete monoclonal antibodies which are complementary to the foreign antigen.
- B memory cells: remain in the body for future immunity (see this article).
This diagram shows how the T-cells and B-cells work together:
Summary
- Foreign antigens trigger an immune response.
- Phagocytosis is a non-specific response where phagocytes engulf and digest pathogens.
- Antigen-presenting cells activate T-cells. Helper T-cells release cytokines to help activate B-cells.
- B-cells divide and differentiate into plasma cells. Plasma cells produce and secrete monoclonal antibodies.
