The most potent stimulator of naive T cells-
Mature B cells and T-cells before antigenic exposure are called naive B and T cells respectively.

The sequence of events in the activation of naive T cells :

Immature dendritic cells in the epidermis are called Langerhans cells.
These immature dendritic cells (Langerhans cells) capture the antigen in the epidermis.
After capturing the antigen, these cells secrete cytokines.
These cytokines cause loss of adhesiveness of Langerhans cells.
Langerhans cells separate from each other and migrate to lymphatic vessels.
In a lymphatic vessel, maturation of Langerhans cells takes place.
Then these mature Langerhans dendritic cells reach to naive T cells in the lymph nodes and present antigen to these cells and activate them.
Mature dendritic cells
Follicular dendritic cells
Macrophages
B cell