11.1 – Defence Against Infectious Diseases

11.1 – Defence Against Infectious Diseases

11.1.1 – Describe the process of blood clotting

Clotting is an important process as it prevents the body from losing too much blood from wounds, as well as preventing pathogens from entering the body and maintaining blood pressure. The process begins by releasing platelets the tissue. The platelets are cell fragments that circulate in the bloodstream with the red blood cells and white blood cells (the erythrocytes and leukocytes).

The clotting factors, which are released from the platelets or damaged tissue, activate prothrombin to become the enzyme thrombin. This is then responsible for catalysing the conversion of soluble fibrinogen into the insoluble protein fibrin. Fibrinogen is constantly found in the bloodstream. Red blood cells become trapped in the fibrin, preventing the flow of blood.

Once platelets reach it, they will produce sticky extensions and connect to each other. As they contract, the liquid is forced out and a clot is formed.

11.1.2 – Outline the principle of challenge and response

Challenge and Response

The principle of challenge and response means that immunity to a specific pathogen is only developed once it is encountered in the body. In other words, the body needs to be challenged by a pathogen. The response begins when the immune system recognises the antigens on the pathogen and begins taking steps to fight it. The white blood cells recognise the antigen, engulf it, and then reform the antigen on their cell membrane. The cell then goes to the lymph nodes, where the B-cells are found.

The white blood cells will locate the B-cell that corresponds to the antigen, and present the antigen to be B-cell. It is possible for more than one B-cell may be required, called polyclonal response. All these B-cells will start cloning to form different types of plasma cell, making different antibodies to fight all of the antigens on the pathogen.

Clonal Selection

This process involves identifying the B-cell that corresponds to the antigen. The B-cell will then clone itself to make plasma cells, which will in turn produce and excrete the appropriate antibodies.

T killer cells, which are cytotoxic cells, are also formed to detect and destroy cells that are infected.

Memory Cells

The body’s memory cells allow it to have long-term protection against diseases, even after the B-cells and antibodies are gone. They are produced from B-cells after clonal selection, so the corresponding pathogen must attack the body before immunity can be formed.

11.1.3 – Define active and passive immunity

Active Immunity – Immunity due to the production of antibodies by the organism itself after the body’s defence mechanisms have been stimulated by antigens.

Passive Immunity – When the antibodies are acquired from another organism in which active immunity has been stimulated, including via the placenta, colostrums or by injecting antibodies.

It is also helpful to know these definitions:

Natural Immunity – Immunity due to infection
Artificial Immunity – Immunity due to inoculation with a vaccine

11.1.4 – Explain antibody production

Our immune systems are able to make a huge range of antibodies to fight against disease, however it is not possible for all of these to remain in the body at once. Instead, the B-cells are stored, which can then begin producing antibodies if they are necessary. The whole process of building up enough antibodies takes only a few days.

The first stage is when the macrophages, or white blood cells, encounter the antigen. They take it in, and then attach it to the MHC proteins in the plasma membrane.

The helper T-cells can recognise antigens using the receptors on their membranes, and bind to the macrophage that carries it. The macrophage will then pass on a signal that causes the helper T-cell to be activated.

The helper T-cells use a similar process to activate the appropriate B-cell. The helper T-cell will locate a B-cell that has antibodies that correspond to the antigen it is carrying. The receptors bind, and the B-cell is activated.

B-cells use the process of clonal selection to produce the required antibodies. If a B-cell encounters their corresponding antigen, they begin cloning themselves to make more B cells, which then produce the antibody to fight off the disease. These clones are called plasma cells. They have a greater volume of cytoplasm and a large amount of rough endoplasmic reticulum for the synthesis of antibodies.

However, it is possible for one antigen to be destroyed by multiple antibodies. As a result, more than one type of clone cell is formed in the process called polyclonal selection.

Once the plasma cells begin producing large amounts of antibodies, which are secreted by exocytosis, the body has become immune to the disease.

Memory cells are formed at the same time as helper T-cells and B-cells, but these last much longer, after the antibodies and activated B-cells have disappeared. Since they remain, they allow the body to have a more rapid response if the same disease attacks the body.

11.1.5 – Describe the production of monoclonal antibodies and their use in diagnosis and in treatment

Monoclonal antibodies are produced in large quantities and can then be used for many purposes. The process is as follows:

  • The antigens that bind to the desired antibody are injected into an animal
  • The animal’s B-cells that produce this antibody are then extracted from the animal
  • Tumour cells are obtained, which will divide endlessly
  • The B-cells and tumour cells are fused to become hybridoma cells. As a result, the cells will continue to divide and grow, producing the antibody as they do so.
  • The antibodies can then be extracted and purified for use

Monoclonal antibodies are used to treat diseases, such as anthrax. This disease is caused by bacteria that secrete poisons. The monoclonal antibodies can be injected to neutralise the toxins until the body has time to respond to the disease and begin producing its own antibodies.

Another application is in the detection of disease, including malaria. The monoclonal antibodies are able to bind to the antigens. By collecting a sample and placing it on a test plate coated in antibodies, the presence of the antigen can be detected. An enzyme is usually added that causes the plate to change colour in the presence of the antibodies bound to the antigens. In addition, it is possible to see the level of infection and distinguish the strain of the disease.

11.1.6 – Explain the principle of vaccination

When we are vaccinated, we are injected with a modified form of the disease-causing microorganism. This initiates a response from the body, even though the disease and its symptoms do not develop. The pathogen is either dead or weakened (or attenuated). Some vaccines come in the form of an inactivated toxin.

Since the microorganism still has the same antigen, it causes the B-cells to be activated and begin cloning. The memory cells then form so that if the pathogen is encountered again, the body can respond quickly and produce antibodies.

Vaccines are usually injected, although some can be ingested. Multiple vaccines sometimes need to be given to produce a sufficient response in the body.

11.1.7 – Discuss the benefits and dangers of vaccination