B cells bearing antibodies and T cells bearing ab or gd receptors recognize the appearance of an invader in the body in different ways. B cell antibodies bind to the invading particle, such as a bacterium, in the form in which it enters the body. The ab receptor-bearing T cells do not bind the invader directly. Instead, they bind to peptide fragments made from the invader's proteins. These fragments are created inside other cells. For example, viruses must invade host cells to increase in number. There, they produce their own proteins and copies of their genes.
Some of the viral proteins, however, are chewed up into peptides by the invaded cells. These peptides bind to proteins called major histocompatibility complex (MHC) proteins. It is this combination of viral peptide and MHC protein that the ab T cell receptors recognize.
It is still not certain how gd receptor bearing T cells recognize invaders. One hypothesis is that they react with damaged cells, that is, rather than recognizing the invader directly, they recognize the damage that the invader has done to the host.
These different ways of homing in on something foreign that has arrived in the body are thought to act as backups for each other. The old systems react with chemicals that are very different between bacteria and mammals. B cells and their antibodies react directly with invading organisms and help rid the body of them. The ab T cells react only with fragments of invading organisms associated with cells. These cells therefore help the body reject organisms, such as viruses or tuberculosis bacteria, that exist inside cells. The ab T cells are also good at reacting with other cells of the immune system, such as B cells. And the gd T cells react with damaged host cells.
Although an invading organism may evolve so that it can avoid one of these methods of recognition, it is almost impossible for an invader to avoid all types of immunity. Therefore, higher vertebrates are well protected against most organisms.