Antibodies (also called immunoglobulins or Ig's) are Y-shaped proteins that circulate through the blood stream and bind to specific antigens, thereby attacking microbes.
The antibodies are transported through the blood and the lymph to the pathogen invasion site.
The body contains millions of different B cells, each able to respond to one specific antigen.
There are 4 classes of antibodies (listed from most common to least common):
Each antibody is made of four polypeptide (protein) chains: 2 heavy chains and 2 light chains. Both heavy chains are identical to each other and both light chains are identical to each other. Each contains a constant region and a variable region. The constant region forms the main part of the molecule while the variable regions forms the antigen-binding site.Each antibody has 2 antigen-binding sites.
Antibodies work in different ways:
1. Neutralizing an Antigen
The antibody can bind to an antigen, forming an antigen-antibody complex. This forms a shield around the antigen, preventing its normal function. This is how toxins from bacteria can be neutralized or how a cell can prevent a viral antigen from binding to a body cell thereby preventing infection.
2. Activating Complement:
Complement is a group of plasma proteins made by the liver that normally are inactive in the body. An antigen-antibody complex triggers a series of reactions that activates these proteins. Some of the activated proteins can cluster together to form a pore or channel that inserts into a microbe's plasma membrane.This lyses (ruptures) the cell. Other complement proteins can cause chemotaxis and inflammation, both of which increase the number of white blood cells at the site of invasion.
3. Precipitating Antigens
Sometimes the antibodies can bind to the same free antigen to cross-link them. This causes the antigen to precipitate out of solution, making it easier for phagocytic cells to ingest them by phagocytosis (as describe above).
Also, the antigens within the cells walls of the bacteria can cross-link, causes the bacteria to clump together in a process called agglutination, again making it easier for phagocytic cells to ingest them by phagocytosis.
4. Facilitating Phagocytosis
The antigen-antibody complex signals phagocytic cells to attack. The complex also binds to the surface of macrophages to further facilitate phagocytosis.