Cytotoxic T Lymphocyte

1. Virus-Infected Cell with Antigen

   • Infected cell displays viral (or abnormal) endogenous antigen via MHC class I molecules.

2. Antigen Presentation

   • Processed antigen presented on surface with MHC class I.

3. T Helper Cell (TH1) Activation

   • TH1 cell binds the MHC I-antigen complex via T cell receptors.

   • This triggers cytokine secretion.

4. Cytotoxic T Lymphocyte (CTL) Activation

   • Cytokines activate CTL precursors → CTLs multiply (clonal expansion).

5. Target Cell Lysis

   • CTLs bind to infected cells and induce apoptosis (programmed cell death).

🔁 Flowchart:

Virus-infected cell (with antigen) 
            ↓
Antigen presented on MHC class I 
            ↓
TH1 cell binds to MHC-antigen complex 
            ↓
TH1 cell secretes cytokines 
            ↓
Cytokines activate CTL precursor → CTL clone forms 
            ↓
CTL binds to infected cell 
            ↓
Infected cell is lysed (apoptosis)

📝 Summary:

The cytolytic activity of T cytotoxic cells (CTLs) is a critical immune response to virus-infected or abnormal cells. The infected cell displays endogenous antigens using MHC class I molecules. T helper (TH1) cells detect this and release cytokines. These cytokines activate precursor CTLs, which then multiply and directly kill infected cells through apoptosis, helping eliminate the infection.