What is the term for an individual's first exposure to an antigen that triggers a primary immune response?

The term used to describe an individual's first exposure to an antigen that triggers a primary immune response is sensitization. During this initial encounter with an antigen, such as a pathogen or allergen, the immune system begins to recognize the antigen and mounts an immune response. This response includes the activation of T and B lymphocytes, which will produce antibodies specific to that antigen.

Sensitization is significant because it establishes immunological memory. Upon subsequent exposures to the same antigen, the immune system can respond more swiftly and effectively, often resulting in a stronger secondary immune response. This foundational knowledge is crucial in understanding how vaccinations work and how the immune system functions in response to infections.

Cellular immunity refers to the immune responses that involve the activation of T cells, which can occur during sensitization but does not specifically define the first exposure. Hypersensitivity relates to exaggerated immune responses and allergies that can occur after sensitization, while passive immunity involves the transfer of antibodies from one individual to another and does not pertain to the individual's primary immune response.