Clonal deletion can occur centrally during the initial differentiation of antigen-specific T cells or B cells or even later in peripheral sites. In the case of T cells, the site of T cell differentiation is the thymus (Sprent and Webb, 1995).

What occurs during clonal deletion?

Clonal deletion is the removal through apoptosis of B cells and T cells that have expressed receptors for self before developing into fully immunocompetent lymphocytes. This prevents recognition and destruction of self host cells, making it a type of negative selection or central tolerance.

Why does clonal selection occur?

Clonal selection is the theory that specific antigen receptors exist on lymphocytes before they are presented with an antigen due to random mutations during initial maturation and proliferation. After antigen presentation, selected lymphocytes undergo clonal expansion because they have the needed antigen receptor.