The immune system uses two main approaches to fight off infection. The first is a general approach known as cell-mediated immunity in which certain immune system cells attack and destroy infectious invaders. The second approach is more specialized and is known as humoral immunity. The cornerstone of humoral immunity is formed by antibodies. Antibodies latch onto infectious invaders such as viruses, bacteria, molds, and parasites and swiftly aid in their destruction.
Antibodies are proteins that belong to a class of protein molecules called immunoglobulins (Ig), also known as gamma-globulins. Immunoglobulins are the most important molecules in the immune system. They have been reported to consist of 19,996 atoms associated in 1,320 amino acid units. The three main groups of immunoglobulins are IgG (a glycoprotein and the immunoglobulin present in the body in the largest concentration), IgA, and IgM. Immunoglobulin G has a molecular weight of about 160,000. The larger chains each have a molecular weights of about 50,000, and the smaller chains molecular masses of about 25,000.
The cells that are involved in humoral (antibody) immunity are B cells and helper T cells. Both the B cells and the T cells belong to a class of immune system cells called lymphocytes. Only the B cells produce antibodies, but they can't release them without the assistance of helper T cells.
When infectious invaders enter the body, the T cells and B cells identify them as foreign owing to their antigens. Antigens are macromolecules, usually proteins having a molecular weight of at least 10,000, on the invader that are unlike molecules normally found in the body, and which trigger the formation of antibodies in the blood. An antibody attacks an invader by binding to an antigen. (In autoimmune diseases, such as rheumatoid arthritis, the immune system confuses a factor in the body with an antigen and launches an attack against the body itself.) The antibody-antigen complex alerts the immune system that there is an invader and other immune system components move quickly to destroy it and stop, or even prevent, disease. Antigens appear to "mate" with antibodies much like enzymes bond to substrates, i.e., in such a way as to match certain radicals (especially polar and quaternary ammonium groups) with complementary structures in the antibody molecule.
The relationship between an antibody and an antigen is very specific as each individual antibody can only recognize a specific antigen. Since the immune system may encounter countless different antigens over a lifetime, it has to be prepared to produce a wide variety of antibodies. This challenge is handled at the genetic level.
Each antibody molecule is composed of four subunits—two identical heavy chains and two identical light chains. Both chains are coded for in three or more sections of DNA, and each section of DNA contains many different genes. There can be more than 300 genes in a section, but only one is used for each antibody molecule. Because there is a large choice of which gene to use from each section, and because the choice is made randomly, the odds are very good that each individual B cell will produce a unique antibody.
Once all four chains are produced and matched up with one another, they form a large Y-shaped molecule. The end of each arm of the Y is called an antigen-binding site; therefore, each antibody has two antigen-binding sites. These antigen-binding sites are presented on the surface of every B cell. If a B cell encounters an antigen to which its unique antibody can bind, it becomes activated. However, it cannot produce and release copies of its antibody until it also encounters an activated helper T cell.
Helper T cells are activated by antigen-presenting cells. Antigen-presenting cells are a type of macrophage--an immune system cell that consumes an infectious invader, processes it, and displays the pieces. Once activated, the number of helper T cells increases greatly. When one of these helper T cells encounters a B cell that has been activated by the same antigen, it secretes cytokines, chemicals that cause the B cell to clone itself. The resulting clones, called plasma cells, are a mature version of the activated B cell. They are able to secrete the antibody which helps the immune system fight infection.
To protect the body against future attacks by the same infectious invader, some of the activated B cells become memory B cells. These memory B cells do not secrete antibody, but they remain in the body for years and provide a swift response to any further encounters with the infection. Because the infectious invader can be recognized so quickly, it does not have the opportunity to cause an infection and a person is said to be immune to the disease.
The antibody produced by the cloned B cells is called a monoclonal antibody. Monoclonal antibodies can also be produced by fusing a single plasma cell with an immortal myeloma cell line in the laboratory. The myeloma cell line contains cells that can be replicated repeatedly but cannot produce antibody. The combined cells are called a hybrid cell line or hybridoma. The hybridoma produces the exact antibody that was created by the plasma cell.
The cells in the hybridoma cell line can be stored and cultured (i.e., grown under suitable conditions), which allows medical researchers to generate monoclonal antibodies in the laboratory. Because these monoclonal antibodies are highly specific and it is known to which antigen they bind, they can be used to identify and possibly treat a specific diseases. Such monoclonal antibodies are capable of identifying the presence of various diseases in from 20-30 minutes (compared to the three to six days required by conventional techniques). Monoclonal antibodies have been used to combat viruses, bacteria, parasites, and some forms of cancer.
This is the complete article, containing 945 words
(approx. 3 pages at 300 words per page).
