Final answer:
The immune system's three main layers of defense include the instant barrier defenses of skin and mucous membranes, the rapid but nonspecific innate immune response with inflammation and phagocytosis, and the specific, memory-based adaptive immune response involving lymphocytes.
Step-by-step explanation:
Three Means of Protection Afforded by the Immune System
The immune system employs multiple strategies to protect the body from pathogens. Among these protective measures, there are three primary levels of defense:
First Line of Defense: Barrier Defenses
The body's first line of defense consists of barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into body tissues.
Second Line of Defense: Innate Immune Response
The innate immune response quickly mobilizes a variety of specialized cells and soluble factors when pathogens breach the initial barriers. This includes mechanisms such as inflammation, phagocytosis by white blood cells, and activation of the complement system.
Third Line of Defense: Adaptive Immune Response
The adaptive immune response is slower but highly specific, targeting specific pathogens via lymphocytes that control the immune response and can "remember" pathogens to mount a faster response upon re-exposure.