Biology Reference
In-Depth Information
Non-specificity in cellular immunity has also been demonstrated
on countless occasions. The receptor carried by a T-lymphocyte is
capable of recognising antigens different from the one that induced
the immune reaction (Amrani
et al
., 2001; Hausmann
et al
., 1999;
Dutoit
et al
., 2002).
4.1.3
Non-specificity in cell signalling
Cells receive various signals from their environment. In bacteria,
chemotactic signals indicate a source of food or danger, while in
multicellular organisms, signals encourage the multiplication or dif-
ferentiation of cells. In these signalling processes, the first step gen-
erally involves the binding of the signal carried by an extracellular
chemical ligand with the extracellular domain of a receptor mole-
cule located in the cell membrane. This binding activates the intra-
cellular domain of the receptor which then triggers a cascade of
molecular interactions inside the cell, transducing the signal (Fig. 8).
The crossed reactions between antigens and T-lymphocyte recep-
tors that we mentioned in the previous section are not exceptional.
Although the cells have to respond precisely to the signals they
received, non-specificity affects the receptor binding to its extracel-
lular ligand just as much as it affects the reactions that transduce
the signal within the cell.
For example, the bacterium
Escherichia coli
uses only four recep-
tors to respond to pH, temperature, and about 50 chemical sub-
stances (Bray, 2003; Ames
et al
., 2002). In the case of the mammalian
epithelial growth factor receptor, at least six different ligands have
been identified (Schweitzer and Shilo, 1997; Carpenter, 2000). In the
same way, chemokines are involved in thymocyte migration and other
cellular functions, particularly the production of blood cells, with
more than 50 chemokines having been identified for only 16 recep-
tors. Each of the 50 chemokines can interact with one or more of
these 16 receptors and, conversely, each receptor can interact with
several chemokines (Broxmeyer and Kim, 1999; Fu and Chen, 2004).
The situation is similar as regards intracellular reactions involv-
ing receptors and the interaction cascades they induce in the cells.
