Cell-Mediated Adaptive Immune Response — Comprehensive Notes

The immune system responds to specific infections by releasing cells that target and destroy infected cells; this is the cell-mediated adaptive immune response, primarily involving T cells.
Macrophages and dendritic cells act as antigen-presenting cells (APCs).
- Macrophages: phagocytose pathogens or pathogen-infected cells.
- Dendritic cells: professional APCs that present antigens on their surface to help coordinate a specific immune response.
- Both cell types present antigens on their surface so that other immune cells can recognize the infection and coordinate the response.
Antigen presentation is a bridge between innate and adaptive immunity, enabling T cells to recognize and target specific pathogens.

T cells develop antigen specificity to target infected cells with a specific pathogen.
- The T cell receptor (TCR) binds to a specific antigen peptide presented by an APC.
- This specificity allows T cells to respond to each pathogen individually.
Once specificity is established, T cells can form a memory response that is triggered on secondary infection by the same pathogen.
This process enables a rapid and targeted response upon re-exposure.

Regulatory T (Treg) cells help suppress the immune response to prevent unintended damage from non-specific immune processes.
They contribute to controlling and terminating immune responses after a pathogen has been eliminated.
This suppression is essential for maintaining immune homeostasis and preventing autoimmunity.
After pathogen elimination, the immune system returns to a dormant state to avoid continuous tissue damage.

Memory T cells persist after an infection is resolved.
Upon re-exposure to the same pathogen, memory T cells rapidly proliferate and differentiate into a large population of effector T cells.
This results in a faster, more robust secondary immune response.

Cytotoxic T cells (CD8+ T cells) recognize and lyse infected host cells.
They target cells displaying foreign antigen peptides on their surface, typically via MHC class I presentation.
Mechanisms of killing include induction of apoptosis or direct lysis of infected cells to stop pathogen spread.

Infected cells display antigen peptides on their surface to be recognized by cytotoxic T cells.
This antigen presentation is crucial for CTL recognition and selective killing of infected cells.
The interaction occurs when TCRs on cytotoxic T cells bind to antigen-MHC complexes on the surface of infected cells.

Step 1: APCs (macrophages/dendritic cells) present antigens to naive T cells.
Step 2: T cell develops antigen specificity and becomes activated.
Step 3: Activated T cells differentiate into various effectors:
- Cytotoxic T cells that lyse infected cells.
- Helper T cells (not detailed in the transcript but typically involved in coordinating responses) and regulatory T cells to modulate activity.
Step 4: Infected cells present antigens to CTLs, enabling targeted killing.
Step 5: Regulatory T cells suppress ongoing responses to prevent tissue damage after clearance.
Step 6: Memory T cells persist, enabling a faster and stronger secondary response upon re-exposure.

Relevance to vaccines: Vaccination aims to generate memory T cells for rapid secondary responses.
Autoimmunity risk: Overactive regulatory mechanisms or insufficient Treg function can lead to excessive tissue damage or autoimmunity.
Infection dynamics: The balance between cytotoxic activity and regulatory control shapes disease progression and recovery.
Real-world example: A new pathogen would be handled first by APCs presenting novel antigens, followed by T cell activation, CTL-mediated killing, regulation to minimize damage, and memory formation for future exposures.

Antigen-presenting cells (APCs): Cells that process and present antigen fragments on their surface to T cells (e.g., macrophages, dendritic cells).
Antigen specificity: The ability of a T cell receptor to recognize a specific antigen.
T cell receptor (TCR): The molecule on T cells that recognizes antigen-MHC complexes.
Regulatory T cells (Tregs): T cells that suppress immune responses to maintain tolerance and prevent autoimmunity.
Memory T cells: T cells that persist after an infection and enable a stronger, faster response upon re-exposure.
Cytotoxic T cells (CD8+ T cells): T cells that kill infected cells presenting antigen on MHC class I.
MHC class I: Molecules that present endogenous antigen peptides to CD8+ T cells.

Explain the roles of macrophages and dendritic cells in initiating the cell-mediated adaptive immune response.
Describe how T cell antigen specificity develops and why memory T cells are important.
Discuss the function of regulatory T cells in preventing immunopathology and how their activity is balanced with effector responses.
Outline the sequence of events from antigen presentation to cytotoxic T cell-mediated killing of infected cells, including the formation of memory.