Final answer:
The body's defense system involves an initial innate immune response with non-specific mechanisms, followed by an adaptive immune response that targets specific pathogens and develops immunological memory.
Step-by-step explanation:
The body’s defense system consists of various phases that operate in a sequential manner to protect the body from harmful agents such as pathogens. The primary phases include the innate immune response and the adaptive immune response. Initially, the innate immune system provides immediate, non-specific defense mechanisms through physical barriers like the skin, chemicals in the blood, and immune system cells that attack foreign cells within the body. If pathogens surpass these defenses, the adaptive immune system is activated, which involves a specific response to the pathogen characterized by the production of antibodies by B-cells and the destruction of infected cells by T-cells.
The sequence of defense thus involves an immediate, rapid response to prevent infection (innate immunity), followed by a more specialized, targeted, and memory-based response (adaptive immunity).