Which theory states that an antigen selects which lymphocyte will undergo clonal expansion?

The theory that describes how an antigen selects which lymphocyte will undergo clonal expansion is known as clonal selection theory. This theory posits that within the immune system, each lymphocyte (specifically B and T cells) has receptors specific to a particular antigen. When an antigen enters the body, it binds to the specific lymphocyte that has the matching receptor. This binding activates the lymphocyte, leading to its proliferation and differentiation into effector cells that combat the antigen.

Clonal selection theory highlights the specificity and adaptability of the immune response, emphasizing that only those lymphocytes whose receptors specifically recognize an antigen will multiply and produce a targeted immune response. This mechanism ensures that the body generates a robust defense against pathogens while maintaining self-tolerance to avoid attacking its own tissues.

In contrast, other theories such as clonal preservation and clonal deletion involve the maintenance and elimination of lymphocytes, respectively, without focusing on the role of antigens in selecting specific lymphocytes for activation. Antigen presentation theory, on the other hand, relates to how antigens are processed and presented to immune cells, which is a crucial part of the immune response but does not directly explain the selection and expansion of lymphocytes in response to an antigen.