Effector cells are a type of lymphocyte that are actively engaged in secreting antibodies. The clonal selection theory stipulates that when naïve cells encounter antigens for the first time, they are stimulated to proliferate and differentiate into effector cells and memory cells.

The specific immunity class cells release the effector cells after being activated by an antigen. When antigens are presented by the class II MHC Helper T-cells will produce effector cells and memory cells. When presented by class I MHC Cytotoxic T-cells along with the activation by cytokines from helper T-cells will produce effector cells and memory cells as well.

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Effector cell". Image Credit.