Overview of the Adaptive Immune Response
The adaptive immune response is a targeted defense mechanism that activates after viral infection, involving specialized lymphocytes that recognize specific viral antigens. Upon infection, viruses enter host cells and replicate, prompting antigen-presenting cells (APCs) like dendritic cells to process and display viral peptides on MHC molecules. This initiates the activation of T cells and B cells, leading to the production of antibodies and direct cell killing to clear the infection.
Key Components: T Cells and B Cells
The response relies on two main cell types: helper T cells (CD4+) and cytotoxic T cells (CD8+), along with B cells. Helper T cells recognize antigens on MHC class II and release cytokines to activate B cells and cytotoxic T cells. B cells differentiate into plasma cells that secrete virus-specific antibodies, which neutralize free viruses by binding to their surface proteins. Cytotoxic T cells identify infected cells via MHC class I-presented antigens and induce apoptosis to prevent viral spread.
Practical Example: Response to HIV
In HIV infection, the adaptive immune system detects viral glycoproteins on infected CD4+ T cells. Helper T cells, ironically depleted by the virus, attempt to coordinate the response, leading to cytotoxic T cells targeting infected cells and B cells producing neutralizing antibodies against the envelope protein gp120. However, HIV's high mutation rate allows it to evade full clearance, illustrating how the response controls but does not always eradicate chronic viruses.
Importance and Applications
This response is crucial for resolving most viral infections and establishing immunological memory, enabling faster secondary responses via memory T and B cells, as seen in vaccinations. It underpins vaccine efficacy, such as in measles or COVID-19 vaccines, where induced adaptive immunity prevents severe disease. Understanding it aids in developing antiviral therapies and immunotherapies for persistent infections like hepatitis.