Recognition of the Immune System
Generating Receptor Diversity
Affinity Maturation
Immune Memory
Tolerance of Self
Eliminating Intracellular Pathogens

Intracellular Pathogens

The immune system is vastly more complex than portrayed in this brief glimpse. Another important facet which is worth mentioning is the problem of intracellular pathogens. Intracelluar pathogens are organisms such as viruses and certain bacteria which live inside host cells. Such pathogens are not visible to B-cells; all that the B-cell can observe is the outside of the host cell, which will look like self. What the body needs is some way of "looking inside" host cells to see if they are infected.

How does the immune system "see" inside host cells?

All cells in the body have a way of collecting fragments of proteins contained within the cell, and transporting them to the surface, where they can be displayed to the rest of the body. The molecules that do the transporting are called MHC molecules. If a virus has infected a cell, MHC can carry viral proteins to the cell surface and present them where the immune system can detect the presence of foreign proteins.

How does the immune system eliminate infected cells?

Figure 10. Killer T-cells eliminate intracellular pathogens.

Intracellular pathogens are eliminated by a specialized type of lymphocyte, called a killer T-cell (see figure 10. Killer T-cells are a subclass of T-cells, meaning that they mature in the thymus, are tolerized via clonal deletion, and do not hypermutate when cloning. Killer T-cells bind not to simple proteins, but to proteins held by MHC molecules. In other words, killer T-cells can only recognize proteins expressed by MHC on the surface of host cells. If a killer T-cell is activated by recognition of a MHC/protein combination, it will kill the infected host cell. There are many ways in which this is done, e.g. killer T-cells can punch holes in cell walls, or secrete chemicals that destroy cell walls, etc.

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An Overview of the Immune System. © 1997 Steven A Hofmeyr

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