2023-11-25 11:00:00
What is the immune system?
The immune system is a network of cells, tissues and organs that work together to protect the body from foreign compounds, whether abnormal, tumor or virus-infected cells.
Dr Sylvain Choquet, Head of the Clinical Hematology Department at Pitié-Salpêtrière: The immune system includes two main types of defenses: innate immunity that any normally constituted person has, including animals, and acquired immunity, which is more targeted and involves antibodies specific to the germs encountered.
What is the difference between innate (or natural) immunity and acquired immunity?
As its name suggests, innate immunity is present from birth. “It’s the one that everyone has and which occurs first when the body comes into contact with a pathogen. It is a security immunity which has no no memory” sums up the specialist.
This immunity does not require prior interaction with the foreign compound or pathogen to be activated.
Unlike innate immunity, acquired (or adaptive) immunity is specific to each pathogen. It develops throughout life in response to exposure to foreign cells or vaccination. The main actors of this immunity are the lymphocytes B et les lymphocytes Twhich are capable of specifically recognizing the antigens of infectious agents.
Mechanism: How does innate immunity occur?
Innate immunity acts through a series of non-specific defense mechanisms that are triggered quickly when faced with an infectious agent.
There are two main types of reaction in innate immunity:
– The complement cascade system : This is probably the oldest form of immunity, which was found in prehistoric bacteria. “It is a system of very complex proteins which activate each other in cascade through a system of enzymes, leading to the destruction of the foreign cell” explains Dr Choquet.
– Innate cellular immunity which involves the different blood cells with specific roles.
The mechanisms of innate immunity can be physical, such as the skin-mucosal barrier that prevents pathogens from entering the body. They can also be chemical, thanks to substances secreted by immune cells, substances called cytokines, or even “mechanical”, as is the case with phagocytosisa process where certain cells, macrophages, engulf and destroy infectious agents.
Another key mechanism is the inflammatory response, which helps attract immune cells to the site of infection and stimulate tissue repair. This inflammatory response is triggered by danger signals emitted by infected or damaged cells, and involves the production of chemical mediators such as cytokines.
Innate immunity also relies on the recognition of molecular patterns specific to pathogens, thanks to receptors called Pattern recognition receptor (PRR). These patterns, called PAMPs, are common to many pathogens, allowing quick and non-specific response.
What cells are involved?
Neutrophils versus bacteria
Neutrophils are a type of white blood cell or leukocyte. They are the most abundant cells of the innate immune system. “These cells are capable of detecting and eliminating pathogens, particularly bacteria,” indicates Dr. Choquet.
Neutrophil white blood cells circulate in the blood and must receive a signal to leave the bloodstream and pass into the tissues, where they primarily play their role. This is where they are the main cellular antibacterial agent. Their lifespan is very short, and they are entirely consumed by their function, when they die they form pus.
Lymphocytes or natural killer (NK) cells regulate the inflammatory reaction
NK (Natural Killer) lymphocytes play a major role in innate immunity. “NK are capable of recognizing and eliminating foreign cells, such as tumor cells or cells infected by a virus,” indicates the specialist. They use inhibitory receptors to develop, mature and distinguish “self” from “non-self”.
NK lymphocytes also secrete cytokines which participate in the regulation of the inflammatory reaction but also in the control of the adaptive immune response. They finally have the ability to destroy cancer cells and pathogens through a process called cytolysis.
Macrophages for phagocytosis
Macrophages – which are a type of phagocyte – are another key player in innate immunity.
Dr Choquet: Macrophages come from blood monocytes, which become macrophages when they pass into tissues. They are present in almost all tissues and are the first to detect the presence of pathogens thanks to their PRR receptors.
They have the capacity to ingest and destroy foreign bodies: this is phagocytosis. Thus, they play a crucial role in the body’s non-specific defense, by eliminating damaged cells and extracellular debris.
How does innate immunity persist?
The prolongation of innate immunity is closely linked to adaptive immunity. Indeed, these two types of immunity constantly interact to ensure optimal protection once morest pathogens.
“In case of repeated exposure to the same pathogen, innate immunity strengthens thanks toacquired immune memory : it is acquired or adaptive immunity” describes the head of department. This is mainly caused by B and T lymphocytes, which, once activated by a specific antigen, are capable of “remembering” this antigen and react more quickly and effectively during a new exposure.
Innate immunity, although it is the body’s first line of defense, is not sufficient to eliminate certain pathogens. This is where it is relayed by adaptive immunitywhich offers a more specific and durable immune response.
How do vaccines work?
Vaccines are designed to act in concert with these two immune systems. They contain weakened or inactive elements of an infectious organism (antigens), which trigger an immune response in the body. To put it simply: they mimic the early stages of an infection, mainly by exposing to the immune system components of the pathogen which are initially recognized by the innate immune system and then adaptive immunity, as during natural infection. Therefore the body “learns” to fight infection without risking complications of the disease.
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