CancerQuest | How the Immune System Sees the World

Our immune system constantly surveys our body checking for invaders, like bacteria and viruses. The system is also able to recognize when normal cells become altered such as cancer cells. Recognition of invaders or altered 'self' involves cooperation between different cells and is tightly controlled.

The exact steps involved in the generation of an immune response are slightly different depending on the type of threat (virus, bacteria, etc.) but in general, our cells recognize small parts of the target, usually protein fragments that are created by the digestion of a larger protein. For example, a bacterium that invades the skin via a wound may be recognized by the proteins on its surface.
A protein or other product (sugar, lipid, etc.) that can be recognized by the immune system and lead to the production of an immune response is known as an antigen.

Some immune cells, including macrophages and dendritic cells, are able to carry these proteins on their surface, like waving a flag! The fragments of proteins (antigens) are 'presented' to the B and T cells and cause those cells to become active. The cells that are able to present antigens are known, understandably enough, as Antigen Presenting Cells or APC.

Individual B cells and T cells each express a single type of receptor molecule on their cell membrane. They do have many copies of that receptor on their surface. These receptors are called B-cell receptors (BCR, or immunoglobulins) and T-cell receptors (TCR). Each of these receptors binds to just one very specific peptide (antigen) from an abnormal cell or foreign object. The expression of a single type of receptor ensures that each lymphocyte is specific for just one antigen. Unlike the cells of the innate immune response, lymphocytes can distinguish between very similar target molecules. There are enought different lymphocytes in the body to recognize more than one billion different peptides! This amazing diversity assures that there are cells that are able to recognize just about any target encountered in our lifetimes.

The specific immune response is divided into two parts, humoral and cellular immunity. Humoral immunity is dependent on the production of specific proteins known as antibodies. Antibodies are produced by B cells. These protein interceptors are are small Y-shaped molecules that circulate in blood and other body fluids. When an antibody bumps into its specific target (antigen) it binds tightly allowing the target to be destroyed or inactivated. Antibodies can

Neutralize toxins
Bind to viruses to prevent their entry into cells
Bind molecules in the bloodstream leading to their clearance
Mark the target for consumption by of the non-specific immune system
Work with other proteins in the body to directly kill bacteria and parasites