Figure 10.

The transition from mostly perforin to FasL cytotoxic T lymphocytes (CTL). The CTL start off as naïve CD8⁺ T cells, which upon activation by antigen-laden antigen-presenting cells, and cytokines develop into dividing CTL blasts that continue to proliferate and differentiate into effector CTL of decreasing sizes. Initially, the responding CTL possess both the perforin and FasL killing mechanisms. As the immune reaction progresses, and antigen becomes limiting, most CTL blasts are pruned (probably by activation-induced cell death); the remaining CTL, now of decreasing sizes, gradually stop expressing perforin, relying mainly on FasL activity. These CTL are not memory cells however.