Which type of immunity is considered the third layer of defense?

Acquired immunity, often referred to as adaptive immunity, is regarded as the third layer of defense in the immune system. This type of immunity develops over time and is characterized by a specific response to pathogens the body has previously encountered. It involves the activation of lymphocytes, particularly T cells and B cells, which recognize and remember specific antigens. This memory allows for a more robust and rapid response upon subsequent exposures to the same pathogen.

The other types of immunity, while important, serve different roles within the immune system. Innate immunity represents the first and second lines of defense, encompassing physical barriers such as skin and mucous membranes, as well as immune responses that are non-specific and immediate, such as inflammation and the activity of various immune cells. Physical barrier immunity specifically pertains to these initial protective measures and does not involve the targeted response that defines acquired immunity. Cell-mediated immunity is a component of acquired immunity that specifically refers to the immune response directed by T cells against infected or cancerous cells, but acquired immunity encompasses both humoral (antibody-mediated) and cell-mediated components.

Thus, acquired immunity is correctly identified as the third layer of defense due to its specialized, adaptive nature and its ability to remember and respond to specific pathogens effectively.