A cell is one of the most basic units of life. A cell membrane is a barrier that helps to protect the contents of a cell from the outside environment and cell recognition is a way for the body to communicate. It is a crucial part of the immune system where several mechanisms help to recognise other cells, self-cells, as well as foreign material, such as bacteria, viruses, and toxins so it understands which to protect and which to destroy!
Cell recognition in the plasma membrane
To understand how cell recognition functions in more detail, we must revisit the cell membrane structure. Every cell has a phospholipid bilayer membrane which encloses the cell's organelles and genetic material. Inside this phospholipid bilayer, molecules are necessary for cell function, including channel proteins that allow substances to pass in and out of the cell, and cholesterol, which reduces the bilayer's permeability to create a stronger concentration gradient between the inside and outside of the cell.
Crucially, each cell membrane also contains cell-identifying molecules, which are found on the outside of the phospholipid membrane and extend into the extracellular space, the space outside the cells. These identifying molecules are often called membrane carbohydrates. This is when carbohydrates are bound to molecules present in the phospholipid bilayer. It is these carbohydrate molecules that extend outwards into the extracellular space.
Figure 1. The cell membrane, Wikimedia Commons.
Carbohydrates in cell recognition
Cell membranes contain carbohydrates, proteins, and lipids. Carbohydrates can be covalently linked to proteins in the cell membrane, forming glycoproteins, or lipids, to form glycolipids. These are known as membrane carbohydrates. These molecules mostly face the outside of the cell, extending into the extracellular space. Membrane carbohydrates are important in cell recognition.
The glyco-prefix means sugar, indicating the presence of carbohydrate molecules in the molecule.
The binding of carbohydrates forms glycolipids to the cell membrane's lipid bilayer. A glycosidic, covalent bond joins the carbohydrate and lipid molecules. Glycolipids act as specific sites for cell recognition outside the cell surface membrane.
Glycolipids are mainly involved in cell-to-cell interactions, where the molecule will bind to a specific complementary carbohydrate or carbohydrate-binding protein on neighbouring cells. Glycolipids are also important in the recognition of host cells by viruses.
Membrane proteins in cell recognition
Glycoproteins are formed when a carbohydrate molecule bonds to a protein molecule. The bond in this molecule is covalent. Glycoproteins are the most common membrane carbohydrates, with almost all membrane proteins being linked to carbohydrates.
A Covalent bond refers to two atoms sharing electrons.
These molecules have many functions in the body. In cell recognition, they protect the cell from binding to pathogens such as a virus, bacteria, or toxins. They also allow for interactions with other body cells through the binding of molecules. Glycoproteins can also act as markers for viruses to identify host cells.
An example is the CD4 glycoprotein found on T cells, which HIV specifically binds to.
The structure of a glycoprotein
Cell recognition and the immune system
Cell recognition is a crucial element in the functioning of the immune system. Cells in the body involved in immune response, such as phagocytes and lymphocytes, must identify the presence of pathogens to defend the body against them. The body has non-specific defence mechanisms, such as the action of phagocytes against pathogens, and specific defence mechanisms, such as the action of T lymphocytes and B lymphocytes. Both of these defence mechanisms use cell recognition.
Phagocytes: A type of non-specific cell that can engulf and digest foreign particles.
Lymphocytes: A white blood cell that is part of the immune system.
Pathogens: Are organisms that cause disease.
Lymphocytes
Lymphocytes are a type of white blood cell that plays a key part in the body's immune response. There are around 10 million lymphocytes present in the body, each with different proteins on its surface that are complementary to proteins found on different pathogens; this allows them to recognise the presence of a pathogen in the body.
Lymphocytes only respond to non-self antigens. This is because lymphocytes with receptors complementary to the body's cells either die or are suppressed when a person is still a foetus.
In the foetus, lymphocytes collide almost exclusively with self-material as the body is protected from infection by the placenta. Some of these lymphocytes will have receptors complementary to the body's cells, and these lymphocytes are suppressed. Therefore, only lymphocytes that might complement non-self material are left in the body.
In the adult body, some lymphocytes, B cells to be exact, are produced in the bone marrow. These lymphocytes initially only come into contact with self-antigens. As in the foetus, any lymphocytes that show an immune response when encountering self-antigens undergo apoptosis, a programmed cell death. Therefore, no clones of these anti-self B cells will appear in the blood; only B cells that might respond to foreign antigens will be present.
Antigen-presenting cells
Antigen-presenting cells are cells that display foreign antigens on their cell surface. An important example of antigen-presenting cells is phagocytes.
Phagocytes are another type of white blood cell involved in the immune response. These cells engulf and digest pathogens using hydrolytic enzymes called lysozymes. These enzymes break down the parts of the pathogen, which can then be absorbed into the phagocyte's cytoplasm. Phagocytes can take the antigens from the broken down pathogen and display these antigens on their cell surface.
This process is crucial in triggering the cell-mediated immune response. T lymphocytes, which carry out the cell-mediated response, will only respond to antigens presented on a body cell hence it is called the cell-mediated or cellular response. Therefore phagocytes are crucial in triggering the cell-mediated immune response, where complementary T cells can bind to the presented pathogens of an antigen.
Cell-to-cell recognition example: organ transplants
The idea of cell recognition is important for understanding the risks associated with organ transplants in humans. Organ transplants are taken from selected donors and then given to patients. However, because transplanted organs come from a different body, the antigens on the cell surfaces of the new organ will be different from the antigens on self-cells in the body. This can cause the immune system to attack the transplanted organ and destroy it, just as it would respond to a pathogen. In other words, the transplant will be rejected.
To avoid this, doctors attempt to 'match' organ donors with patients so their tissues and antigens are as similar as possible. Often, this means selecting donors who are genetically similar to the recipient - the best matches often come from relatives. Recipients are also prescribed immunosuppressant drugs, which help prevent the immune system from reacting against the transplant.
Cell Recognition - Key takeaways
- Cell recognition is how body cells communicate to recognise each other or recognise foreign material in the body.
- This recognition is achieved by the presence of identifying molecules on the cell's surface membrane. These molecules have specific 3D shapes which identify the cell, toxin, or viral particle.
- These identifying molecules are commonly membrane carbohydrates; they are molecules found in the cell membrane where a carbohydrate has bound to proteins or lipids in the cell membrane.
- Glycoproteins are the most common form of membrane carbohydrates and are created when a carbohydrate is covalently linked to a membrane protein. Glycolipids are formed when a carbohydrate is covalently linked to a lipid in the phospholipid bilayer of the cell membrane.
Proteins are especially useful for identifying molecules because of their complex 3D structure, forming highly specific shapes.
Cell recognition is vital in the functioning of the immune system. Lymphocytes can recognise the presence of non-self material in the body due to receptors on their cell surface. Phagocytes can present the antigens of pathogens on their cell surface, triggering the cell-mediated response when interacted with by T cells.
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