Elimination of Intracellular Microorganisms

Hello everyone,

 The first  cellular strategy to fight against bacteria have already been discussed  . You can find first strategy for Elimination of Extracellular bacteria.
Today the topic which I would like to share with you is second  the strategy to Eliminate Intracellular Microorganisms. 

The immune system is vastly more complex than it looks. 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.

Even then antibody and Complement opsonise certain bacteria and phagocytosis occur, they are not killed but survive and multiply in Macrophages eg Mycobacterium tuberculosis.

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?

Intracellular pathogens are eliminated by a specialized type of lymphocyte, called a killer T-cell (Helper T Cell). These cell recognise macrophages containing intracellular bacteria by means of T cell antigen receptor (MHC), which is not antibody. 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. they help Macrophages to kill bacteria by synthesizing soluble molecule (Cytokins) which stimulate bacterial killing mechanisms of macrophages. 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.

with love
-Dixy