Introduction to the Immune system

The main building blocks of body tissue are called proteins. The word protein comes from the Greek “Prota” meaning “of primary importance”. Proteins dictate the colour of or eyes, our skin, our hair, the way we think and everything we do! The blueprints for how to build proteins are written or ‘encoded’ in our genes. Proteins are molecular machines that come in many different shapes and sizes; they are formed by folded strings of molecules called amino acids. There are 20 amino acids, and they are the essential components of all proteins and therefore of all life on earth.

Proteins are molecular machines that come in many different shapes and sizes formed by folded strings of aminoacids. There are 20 amino acids, and they are the essential building blocks of all proteins and therefore of all life on earth.

The immune system is a highly evolved organization of specialised cells and molecules that defend our body against cellular abnormalities and infections. MHC molecules play an important role in this machinery by presenting peptides to specialised cells of the immune system called T cells (or T lymphocytes). Usually, the peptides displayed on MHC molecules will be from ‘self’ proteins and these should be ignored by the immune system. However, when a cell becomes cancerous or is infected by a germ it will express different proteins and display different or ‘foreign’ peptides on its surface. T-cells constantly patrol our bodies and scan for other cells that display ‘foreign’ peptides that indicate a cell is infected with a germ or has become cancerous. T-cells are able to see inside infected or transformed cells by scanning their surface with a molecule called the T-cell receptor .

What are T-cells?
T-cells are a type of white blood cell that circulate around our bodies, scanning for cellular abnormalities and infections.

What is the role of T-cells in the immune system?
T-cells are essential for human immunity. The devastating effects of a lower than normal number of just one type of T-cell are all too evident in HIV/AIDS. There are several different kinds of T-cell; broadly speaking they can be divided into two different types, cytotoxic (or killer) T-cells and helper T-cells. Killer T-cells are characterised by the expression of the CD8 co-receptor and are sometimes called “CD8 T-cells”. Killer T-cells scan the peptides displayed on the surface of all other body cells. As these peptides are derived from the all the proteins inside the cell this ingenous system gives killer T-cells ‘X-ray vision’. If killer T-cells find a foreign peptide that should not be there (e.g. a peptide derived from a viral protein or a protein only expressed by cancer cells) they kill the target cell (the clue is in their name!;). Killer T-cells thereby act as the assassins of the immune system and they must be very tightly controlled! The other main type of T-cells are called helper T-cells. Helper T-cells are characterised by the expression of the CD4 co-receptor and orchestrate and coordinate the immune response. In the warfare analogy often used to describe immune defence, then CD4 T-cells are the Generals. You can find out more about these cells using the links to the right.

Why is it important to study T-Cells?
Almost every aspect of the adaptive immune response is controlled, in some way, by T cells. These multifunctional cells have the ability to:
• Scan the intracellular environment for foreign invaders
• Directly kill virally or bacterially infected cells
• Naturally eradicate cancer cells
• Activate and help other immune cells that ingest germs or that make anti-infection molecules called antibodies
• Remember a germ that they encountered decades ago

T cells are also responsible for immune responses that lead to:
• Rejection of a transplanted organ
• Virtually all autoimmune disease (diabetes, multiple sclerosis, rheumatoid arthritis etc.)
• Some allergic reactions (gluten intolerance, allergy to insect stings etc.)