24.9
Vaccinations
Aperçu
La vaccination est l’administration de matériel antigénique provenant d’agents pathogènes pour conférer une immunité contre un micro-organisme…
- La vaccination est l'administration d'un vaccin,le matériel antigénique de pathogènes,qui prépare le corps à de futures infectionspar un pathogène précis. Lorsqu'un vaccin est administré pour la première fois,une cellule présentatrice d'antigène, ou CPA,comme une cellule dendritique,engloutit l'antigène du vaccin, le dégrade,et en affiche des morceaux à sa surface. La CPA active ensuite les lymphocytes T auxiliaires, ou Th,qui activent les lymphocytes T cytotoxiques ou cellules Tc,aussi appelées cellules tueuses,et les lymphocytes B.
Une fois activées,les cellules Tc peuvent détecter les cellules infectées,se lier à elles, et libérer des composés chimiquesqui tuent ces cellules avec les pathogènes qu'elles portent. Après avoir été activés par les Th,les lymphocytes B se divisent entre plasmocyteset cellules B à mémoire. Les plasmocytes créent des anticorpsqui se lient aux antigènes introduits par le vaccinpour les détruire et les éliminer du corps.
Après que la fausse infection induite par le vaccina été maîtrisée,certains cellules Tc sont converties en cellules à mémoirequi peuvent facilement se diviser et croîtredès qu'une réexposition au même pathogène ou à ses antigènesa lieu par la suite. Les cellules à mémoire et anticorps générés par les vaccinscontre un pathogène précisrestent dans le corps pendant des mois ou des années,selon le vaccin. Lorsqu'un individu est infecté par le pathogène ou antigènecontre lequel il a été vacciné,les cellules à mémoire reconnaissent l'antigèneet peuvent se diviser rapidementpour produire une réponse immunitaire plus rapide.
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Q1: How do antigen-presenting cells activate the immune system after vaccination?
Antigen-presenting cells (APCs) such as dendritic cells engulf vaccine antigens, degrade them, and display antigen pieces on their surface bound to MHC molecules. APCs then migrate to lymph nodes where they stimulate naive helper T cells and cytotoxic T cells. This activation triggers cell-mediated immune responses that prepare the body to recognize and eliminate the pathogen if encountered later.
Q2: What is the difference between plasma cells and memory B cells after vaccination?
After activation by helper T cells, B cells differentiate into two types. Plasma cells immediately produce antibodies specific to vaccine antigens that bind to and destroy pathogens. Memory B cells persist in the body for months or years, enabling rapid antibody production upon future exposure to the same pathogen, providing long-term immunity.
Q3: How do memory cells provide faster immune protection during reinfection?
Memory T cells and B cells generated during vaccination recognize antigens from previously encountered pathogens. Upon reexposure, these memory cells divide rapidly and mount a quicker, more efficient immune response than the initial vaccination. This accelerated response prevents or reduces disease severity when the actual pathogen is encountered.
Q4: What role do cytotoxic T lymphocytes play in vaccine-induced immunity?
Cytotoxic T lymphocytes (CTLs), also called killer T cells, are activated by helper T cells after vaccination. CTLs detect infected cells, bind to them, and release chemicals that kill the cells along with harbored pathogens. This cell-mediated response is crucial for eliminating virus-infected cells and preventing pathogen spread.
Q5: How does herd immunity protect unvaccinated individuals in a community?
Herd immunity occurs when a high percentage of the population is vaccinated against a pathogen, preventing its transmission throughout the community. Even unvaccinated individuals, including those too young or immunocompromised to receive vaccines, have reduced infection risk because the pathogen cannot find sufficient susceptible hosts to propagate.
Q6: Why does the influenza vaccine need to be reformulated annually?
The influenza virus evolves at an exceptionally rapid rate, with new viral strains emerging frequently. Because existing vaccines target specific viral strains, a new vaccine must be developed each year to match the currently circulating influenza variants and provide effective protection against infection.
Q7: What types of antigens are used in different vaccine formulations?
Vaccines contain antigens derived from specific pathogens in different forms. Inactive vaccines contain intact but non-replicating antigens, while subunit vaccines contain only pathogen fragments. Attenuated vaccines use live weakened pathogens that stimulate immune responses without causing severe disease, and many vaccines include adjuvants to enhance immune responses.