Antibodies
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Explainer Video
Summary
Antibodies
In a nutshell
Antibodies are important proteins used by the immune system as a defence mechanism against foreign antigens. The production of antibodies can be manipulated by scientists for use in medicine. Antibodies can be used in pregnancy tests, cancer treatments and ELISA tests.
Structure
Antibodies are produced by plasma cells. Plasma cells secrete antibodies that are specific to the antigen and form an antibody-antigen complex.
Antibodies are proteins made up of four polypeptide chains: two shorter light chains (shown in yellow: VL and CL) and two longer heavy chains (shown in blue: VH, CH1, CH2 and CH3). They join to form a Y-shaped protein. The binding site where the antibody binds the antigen is known as the variable region, this unique binding site is complementary to the antigen due to the tertiary structure of the antibody. All antibodies have the same constant region.
Destruction of antigens
Antibodies do not destroy antigens directly: they prepare them for destruction by other immune cells. One way antibodies do this is through the agglutination of bacterial cells. This causes large clumps of bacterial cells to form which makes it easier for phagocytes to locate and destroy them by phagocytosis. Another way antibodies do this is through opsonisation. This is the process by which the pathogen is marked for phagocytosis by antibodies.
Monoclonal antibodies
Definition
Monoclonal antibodies are identical antibodies made by identical immune cells.
Hybridoma cells
Monoclonal antibodies are produced from lots of clones of a single B lymphocyte. This means they are all identical and will only target one specific protein antigen. However, these lymphocytes do not divide very easily. On the other hand, tumour cells don't produce antibodies but they do divide very easily. Therefore, scientists fuse a mouse B lymphocyte with a tumour cell to create a cell called a hybridoma. Hybridoma cells can be cloned to produce lots of identical cells that will all produce monoclonal antibodies. These antibodies can then be collected and purified.
Treating diseases with monoclonal antibodies
Different cells in the body have different antigens on their surface. This means you can make monoclonal antibodies that will bind to specific cells in the body.
Cancer cells have antigens on their cell membranes that are not found on normal body cells - they're called tumour markers. Monoclonal antibodies that bind to tumour markers can be used to treat cancer.
An anti-cancer drug can be attached to these monoclonal antibodies. This could be a radioactive substance, a toxic drug or a chemical that stops the cancer cells from growing and dividing. The antibodies are given to a patient through a drip, they only target the specific cancer cells because they only bind to tumour markers. This means that the drugs kill the cancer cells but not any normal body cells near the tumour.
Treating cancer
Pregnancy tests
Monoclonal antibodies can be used in pregnancy tests. A hormone called human chorionic gonadotrophin (hCG) is produced by the placenta in the early stages of pregnancy. Pregnancy stick tests detect this hormone.
| 1. | The part of the stick you urinate on. |
| 2. | Antibodies specific to hCG with blue beads attached. |
| 3. | The test strip (the part that turns blue if you're pregnant). |
| 4. | Antibodies are stuck down. The test strip has more antibodies specific to hCG stuck onto it. |
| 5. | Hormone (hCG) bound to antibody that is attached to a bead. |
| 6. | The beads are carried in a flow of liquid towards the test strip. |
| 7. | If you're pregnant, the hormone binds to the antibodies with the blue beads attached and the urine moves up the stick carrying the hormone and the beads. The beads and hormones bind to the antibodies on the test strip. The blue beads get stuck on the strip turning it blue. |
| 8. | If you're not pregnant, the urine still moves the blue beads to the test strip, but as there is no hormone, nothing sticks to the test strip so it doesn't go blue. |
ELISA test
The enzyme-linked immunosorbent assay (ELISA) test is used to detect antibodies against a specific antigen or antigens to a specific antibody in the blood of a patient. It is a widely used test as it can detect pathogenic infections and allergies.
In an ELISA test, an antibody is bound to an enzyme. This enzyme reacts with a specific substrate to produce a coloured compound, therefore if the solution changes colour this demonstrates that the antigen or antibody of interest is present in the sample. Some ELISA tests are quantitative, this means the intensity of colour can be read by a machine called a spectrophotometer. It reads a certain wavelength of light and produces a reading which indicates how much antibody/antigen there is.
There are many different kinds of ELISA. A direct ELISA uses a single antibody that is complementary to the antigen of interest. An indirect ELISA test uses two different antibodies and is explained below.
| 1. | The HIV antigen (A) is bound to the bottom of the wells in a well plate. This is a plastic tray with many circular wells in it. |
| 2. | The patient's blood plasma is added to the well. Any antibodies (B) that are specific to HIV will bind to the HIV antigens stuck at the bottom of the well (C). These antibodies are known as primary antibodies. |
| 3. | The wells are then washed out to remove any unbound primary antibodies. |
| 4. | A secondary antibody (D) that has an enzyme attached and is specific to HIV antibodies is added to the well (E). |
| 5. | The well is then washed again to remove any unbound secondary antibody. |
| 6. | A solution containing the substrate specific to the enzyme bound to the secondary antibody is added to the wells. If the solution changes colour (F) this means there must be HIV antibodies in the sample and the person is HIV-positive. If there is no colour change this is because all of the unbound antibodies have been washed away because the person has no HIV antibodies and is HIV-negative. |
Ethical issues
Animals such as mice are used to produce monoclonal antibodies. This raises ethical issues as the mice are deliberately given cancer using tumour cells. Despite the detailed guidelines drawn up to minimise suffering, people still disagree with using animals in this way. Whilst it is true that monoclonal antibodies have been used to successfully treat diseases like cancer, some monoclonal antibody trials have led to deaths and therefore the risks must be fully communicated to the patient before the trial.
FAQs - Frequently Asked Questions
What is opsonisation?
Opsonisation is the process by which the pathogen is marked for phagocytosis by antibodies.
What is a hybridoma?
A mouse B lymphocyte fused with a tumour cell is called a hybridoma.
What does ELISA stand for?
ELISA stands for enzyme-linked immunosorbent assay.
What is a monoclonal antibody?
Monoclonal antibodies are identical antibodies made by identical immune cells.