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Glossary

You will find here several definitions of technical terms used throughout our website.

Antibody

Also called immunoglobulin, an antibody (Ab for short) is a large protein used by the immune system to fight off pathogens by recognizing specifically the antigen they are directed against.
Shaped like a “Y”, antibodies are mainly produced by a category of immune cells called Plasma Cells. They are equipped with two antigen binding sites at both ends of the “Y”, allowing them to latch on to the pathogen and neutralize it.
Because they are so efficient at recognizing a particular target, antibodies have found a wide range of uses in biochemistry, from targeted immunotherapies to protein purifying processes in laboratories.
Monoclonal Antibodies are antibodies that target the same antigen, and that are obtained by cloning a parent, antibody-producing cell.

Antigen

An antigen designates any molecule or compound that triggers a response by the immune system. For example, an antigen can be a marker on the surface of a bacteria, a toxin, an allergen, or an abnormal protein on the surface of a human cell.
The innate immune system will react the same way to all antigens, but the adaptive immune system – as the name indicates – will build a different and highly efficient response to each one.

Apoptosis

Also called programmed cell death, apoptosis is a genetically encoded process by which a cell will induce its own death, with minimal impact on the rest of the body. This process is very prevalent in mammals and allows the body to regulate the cell populations in tissues, control the scale of an immune response, eliminate malfunctioning cells, etc.
In normal cells, DNA damage may trigger apoptosis (as the cell cannot function properly), but cancer cells often lose the ability to undergo this mechanism.

CAR-T cells

Chimeric Antigen Receptors (CARs) are artificial receptors, tailor-made to target certain tumoral antigens. T cells, extracted from a cancer patient and equipped with these antigens, are known as CAR-T cells, or CAR-Ts. They are then re-injected to the patient to help fight the tumor.

HLA

In humans, the MHC – see definition below – is also called the HLA (Human Leukocyte Antigen) complex.

Immune checkpoints

Immune checkpoints (ICPs for short) are safeguards mechanisms, built into immune cells to make sure the immune system doesn’t mistakenly target healthy cells.
When an ICP is stimulated, for example when confronted with a bacteria, the immune response is activated and the target is attacked. When an ICP is inhibited, for instance when interacting with a normal cell, the immune response is not triggered and the cell is left alone.
Cancer cells often neutralize ICPs, thus avoiding attacks by the immune system.

Immune checkpoint inhibitors

Immune checkpoints inhibitors (ICIs for short) are therapeutic molecules, typically antibodies, which target and block the interaction between immune checkpoints and suppressing molecules.
This way, when a cancer cell produces an immuno-suppressive ligand, the ligand cannot find its target. Thus, the tumor cell will fail to block the immune response.

Immune system

The immune system consists in all the mechanisms that a living body uses to distinguish between its own cells and the “non-self”, invading organisms and rogue cells, and to eliminate non-self intruders.

Lymphocyte

Lymphocytes are immune cells, produced in the bone marrow, the spleen and the lymph nodes, that participate in the adaptive immune response. Each lymphocyte is specific to an antigen, allowing a highly targeted defense.
There are two main families of lymphocytes. Lymphocyte B cells (or simply B cells) can morph into plasmocytes and produce antibodies targeted against their antigen. Lymphocyte T cells (or T cells) assist the immune response either by directly destroying infected or dangerous cells, or by stimulating the activity of other immune cells.

MHC

The Major Histocompatibility Complex (MHC) is a large protein structure on the surface of a cell, whose role is to identify this cell to the immune system, to display or recognize proteins, and to communicate outside the cell. Each cell may express several thousand MHCs on its surface.
There are two categories of MHC in humans: the MHC-I is present on all cells, while the MHC-II is exclusive to certain immune system cells. Generally, immune cells are activated when they recognize an antigen expressed by a MHC.

Proteasome

The proteasome is a large protein structure found within all eukaryotic cells which is tasked with degrading proteins, through a process called proteolysis. Degraded proteins are broken down into small amino-acid chains called peptides. A portion of these peptides are then expressed at the surface of the cell (through the MHC complexes), giving the immune system an idea of the cell’s activity.
Proteins are degraded for a number of reasons. They may be dysfunctional or damaged, or the cell may not need them anymore. Additionally, cells can sometimes force the destruction of invasive agents, such as bacterial proteins.

T cells

A subtype of the lymphocyte family, T cells are called this way because they are mainly produced by the thymus. Their role is to recognize antigens, thanks to a highly variable receptor known as the T Cell Receptor (TCR). Each T cell possesses one shape of TCR, and a TCR recognizes one specific antigen; thus, each T cell is specific to a unique antigen.
There are two major types of T cells: the helper T cells, which regulates the activity of other immune cells, and the cytotoxic T cells, which are tasked with destroying other cells when they present the antigen recognized by their TCR. Helper T cells are often called CD4+ T cells, because they carry the CD4 complex, a glycoprotein which assists the Helper’s T Cell Receptors (TCR) interact with Antigen-Presenting Cells. For similar reasons, Cytotoxic T cells are also known as CD8+ T cells, as they carry the CD8 complex.

Telomerase

Telomerase is an enzyme, expressed in humans only in cancer cells, that compensates the shortening of the telomeres with each cell generation.
Every time a cell divides itself to create two new cells (a process called mitosis), it must first replicate its DNA into two identical copies. The way our DNA is replicated means that with each cell generation, the edges of our chromosomes get slightly shorter. This is why their end parts are composed of telomeres, regions of repetitive, non-coding DNA, which our chromosomes can afford to lose.
For normal cells, a large number of division will eventually deplete the telomeres, meaning more replication cycles would damage coding DNA and result in malfunctioning proteins. Thus, the now-« old » cell will die through a process call apoptosis (programmed cell death).
Cancer cells, on the other hand, will produce the telomerase enzyme, which adds non-coding DNA to the telomeres at the end of each replication cycle. Consequently, the telomeres never get shorter, and coding DNA is protected. This is why cancer cells are called « immortal »: they do not get older and can replicate an infinite number of times, in the proper conditions.