Understanding The Immune System
|
 |
In order to recognize and respond to the antigens that are their specific targets, both B cells and T cells carry special receptor molecules on their surface. For the B cell this receptor is a prototype of the antibody the B cell is prepared to manufacture, anchored in its surface. When a B cell encounters a matching antigen in the blood or other body fluid, this antibody-like receptor allows the B cell to interact with it very efficiently. The T cell receptor is more complex. Structurally it is somewhat similar to an antibody, made of a:pair of chemically linked chains with variable and constant regions. (But to work it needs the help of an associated set of cell surface molecules called T3.) Unlike a B cell, however, a T cell cannot recognize antigen in its natural state; the antigen must first be broken down, and the fragments bound to an MHC molecule, by an antigen-presenting cell. Helper T cells (T4 cells) look for antigen bound to a class11 MHC molecule-a combination displayed by macrophages and B cells. Cytotoxic T cells (T8 cells), on the other hand, respond to antigen baund to MHC class I molecules, which are found on almost all body cells. The T cell receptor molecule thus forrns a three-way complex with its specif~c foreign antigen and an MHC protein. This complicated arrangement assures that T cell which affect other cells through either direct contact or bursts of secretions-act only on precise targets and at close range. |
|
Antigen receptors |
Mounting an Immune Response Infections remain the most common cause of human disease.
Produced by bacteria, viruses, parasites, and fungi, infections
may range from relatively mild respiratory illnesses such as
the common cold, to debilitating conditions like chronic hepatitis,
to life-threatening diseases such as AIDS and meningitis. |
