Cell recognition is an important way that cells talk to each other. It helps our bodies know which cells are safe and which ones aren't. This is especially important for our immune system, which protects us from harmful bacteria, viruses, and toxins. To do this, cells have special molecules on their surface that interact with other cells. These molecules are unique for each type of cell, so our body can tell the difference between cells that belong and ones that don't. Some of these molecules are called glycolipids and glycoproteins. Proteins on the surface of cells are especially good at helping with recognition because they have different shapes based on their amino acid sequence. Scientists call these identifying molecules antigens, especially when talking about how our immune system responds to them.

The cell membrane and cell recognition

To learn more about cell recognition, we need to look at the structure of the cell membrane. Every cell has a membrane made of phospholipids, which is like a protective wall around the cell's important parts. The membrane has special molecules that help the cell function, like channels that let things in and out, and cholesterol that makes the membrane stronger. But what's really interesting is that each cell has its own unique set of identifying molecules on its membrane. These molecules are called membrane carbohydrates, and they stick out into the space outside the cell. They're called carbohydrates because they're made of sugar molecules that are attached to other molecules in the membrane.

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Membrane carbohydrates

Cell membranes have carbohydrates, proteins, and lipids. Carbohydrates can be attached to proteins or lipids to create glycoproteins and glycolipids, which are also called membrane carbohydrates. These molecules mostly stick out of the cell and into the space outside of it. Membrane carbohydrates are really important for cell recognition. The word "glyco" means sugar, so when you see it in a molecule name, it means there are sugar molecules in there.

Glycoproteins

Glycoproteins are made when a carbohydrate molecule and a protein molecule bond together. This bond is called a covalent bond, which means that the two atoms share electrons. Glycoproteins are really common in cell membranes, and almost all membrane proteins have carbohydrates attached to them.

These molecules have lots of different jobs in the body. One of their most important jobs is to help with cell recognition, which means they protect the cell from pathogens like viruses, bacteria, and toxins. They also help cells interact with each other by binding to other molecules. Glycoproteins can even act as markers that viruses use to find and infect host cells.

For example, the CD4 glycoprotein found on T cells is really important for HIV to infect those cells. The structure of a glycoprotein is really complex and can vary depending on the specific protein and carbohydrate involved.

Glycolipids

Glycolipids are created when a carbohydrate molecule bonds to a lipid molecule in the cell membrane's lipid bilayer. This bond is called a glycosidic bond and it's covalent. Glycolipids are important for cell recognition on the outer surface of the cell membrane. They're 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.

Cell recognition is a really important process in the immune system. Cells involved in immune response, like phagocytes and lymphocytes, have to identify the presence of pathogens to defend the body against them. The body has both non-specific defence mechanisms, like the action of phagocytes against pathogens, and specific defence mechanisms, like the action of T lymphocytes and B lymphocytes. Both of these defence mechanisms use cell recognition.

Phagocytes are a type of non-specific cell that can engulf and digest foreign particles. Lymphocytes are a type of white blood cell that are part of the immune system. Pathogens are organisms that can cause disease.

Lymphocytes

Lymphocytes are a really important type of white blood cell that play a key role in the immune response. There are around 10 million lymphocytes 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 own cells either die or are suppressed when a person is still a fetus. During fetal development, lymphocytes primarily come into contact with self-material as the body is protected from infection by the placenta. Lymphocytes with receptors complementary to the body's own cells are suppressed, so only lymphocytes that might complement non-self material are left in the body.

In adults, some lymphocytes, specifically B cells, are produced in the bone marrow. These lymphocytes initially only come into contact with self-antigens. As in the fetus, any lymphocytes that show an immune response when encountering self-antigens undergo apoptosis, a programmed cell death. This prevents clones of anti-self B cells from appearing in the blood, and only B cells that might respond to foreign antigens are present.

Antigen-presenting cells

Antigen-presenting cells are cells that display foreign antigens on their cell surface. Phagocytes, another type of white blood cell, are an important example of antigen-presenting cells. Phagocytes engulf and digest pathogens using hydrolytic enzymes called lysozymes, and can display the antigens from the broken down pathogen on their cell surface. This process is crucial in triggering the cell-mediated immune response, where complementary T cells can bind to the presented antigens.

Cell recognition is also important when it comes to organ transplants in humans. Transplanted organs come from a different body, and so the antigens on the new organ's cell surfaces will be different from the antigens on the recipient's own cells. This can cause the immune system to attack and reject the transplanted organ, just as it would respond to a pathogen. To avoid this, doctors attempt to match organ donors with patients so their tissues and antigens are as similar as possible. This often means selecting donors who are genetically similar to the recipient, with the best matches often coming 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 cells in the body communicate to recognise each other or identify foreign material. This recognition is achieved through identifying molecules on the cell's surface membrane. These molecules have specific 3D shapes that identify the cell, toxin, or viral particle. Membrane carbohydrates are the most common identifying molecules, often covalently linked to membrane proteins to form glycoproteins, or covalently linked to lipids to form glycolipids in the cell membrane.

Proteins are especially useful for identifying molecules due to 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, while phagocytes can present the antigens of pathogens on their cell surface, triggering the cell-mediated response when interacted with by T cells. Understanding cell recognition is important in understanding how the immune system works and how the body can distinguish between self and non-self cells.

Cell Recognition

What is cell recognition?

Cell recognition is the interaction between cells in the body that allows them to distinguish self-cells from non-self material, as well as identify abnormal body cells.

What is involved in cell recognition?

Each cell in the body has identifying molecules, such as glycolipids and glycoproteins, on their cell surface. These molecules have specific structures which identify them as belonging to the body. Different cells can bind to each other using these molecules and interact.

Why is cell recognition important?

Cell recognition is important in the immune system. Cells must be able to distinguish between self-material and non-self material, which may include pathogens and toxins. If cells involved in the immune response can identify non-self material, then they will be able to target pathogens and destroy them before they can significantly harm the body.

What do cell recognition proteins do?

Cell recognition proteins are found on cell surface membranes. They extend into the extracellular space and identify cells as either self or non-self with their specific tertiary structure.

What happens after the recognition of dead cells by phagocytes?

Phagocytes engulf the dead cells and digest them via lysozymes (hydrolytic enzymes).

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