Defence Against Pathogens
Human Biology (Year 12)
How does antibody-mediated immunity work?
The humoral response involves the production and release of antibodies into the blood and lymph, otherwise known as antibody-mediated immunity. This type of immunity provides resistance to viruses, bacteria and bacterial toxins before these substances or micro-organisms enter the body’s cells
Lymphoid tissue contains thousands of types of B-cells, and each type has a receptor for a particular antigen and has the ability to respond to a specific antigen. When an antigen-presenting cell presents the antigen to the specific B-cells, the B-cells are activated.
The antigen is also present to helper T-cells, leading to the release of cytokines, small proteins that are released in response to antigens and act as messengers in the immune response – these cytokines cause the helper T-cells to clone themselves to release different cytokines, which activate the B-cells.
When the B-cells are activated, they enlarge and divide into a group of cells called a clone – most of the clone becomes plasma cells, which secrete the specific antibody capable of attaching to the active site of the antigen. These antibodies circulate in the blood, lymph and extracellular fluid to reach the site of the invasion of micro-organisms or foreign materials.
The remaining B-cells of the clone become memory B cells, which spread to all body tissues to allow for the response to occur more rapidly, should the antigen enter the body again.
On the first exposure to an antigen, the immune reaction is called the primary response. The body’s immune system usually responds fairly slowly, often taking several days to build up large amounts of antibodies, and this is as it takes time for the B-cells to multiply and differentiate into plasma cells and then secrete antibodies.
Once the level of antibodies reaches a peak, it begins to decline – however, the primary response leaves the immune system with a memory of that particular antigen. With a second or subsequent exposure to the same antigen, the response is much faster due to memory cells recognising the antigen more quickly.
With this secondary response, plasma cells are able to form very quickly, with antibody levels in the blood plasma rising rapidly to a higher level that lasts longer. Frequently, this response is so quick that the antigen has little opportunity to exert any noticeable effect on the body and as a result, there is no illness.
How do antibodies work?
Different antibodies protect the body through different methods, and these may include:
Inactivation of foreign enzymes or toxins by combining with them or inhibiting their reaction with other cells or compounds
Bind to the surface of viruses and prevent them from entering cells
Coat bacteria so that they are more easily consumed by phagocytes
Cause particles such as bacteria, virus or foreign blood cells to clump together through a process called agglutination
React with soluble substances to make them insoluble and thus more easily consumed by phagocytes
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