The immune system's lines of defense are comprised of three major components: the innate, the adaptive, and the acquired immune systems. Each system is created separately and works both as an independent defense system and a symbiotic system working in conjunction with the others. Systems are layered in such a way that if threats slip past one line of defense, the next in line is triggered to respond. Of the three components making up the immune system's lines of defense, each one serves in either a defensive or offensive capacity against pathogens like viruses, bacteria, and other foreign bodies.

Every human being is born with an innate immune system. As an integral part of the immune system's lines of defense, innate immunity is also known as a non-specific immunity, owing to its more passive, generalized role in the immune system. Physical barriers, such as the skin and mucous membranes, are the primary component of the innate immune system. These are the body's first lines of defense, preventing pathogens from getting inside the body in the first place. Innate immunity is considered a defensive member of the immune system team, using tools such as inflammation to signal other immuno-related systems to mount a response.

Next in line to defend the body against pathogens is the adaptive immune system. Should bacteria, viruses, or other foreign bodies get past the body's physical barriers, certain cells known as natural killer or T cells attack pathogens directly. When the body signals the presence of toxins, bacteria, damaged body cells, viruses, or other foreign molecules, killer cells and their associated helper cells seek out and destroy the threat before it has a chance to create illness, disease, or abnormal growth. Adaptive immunity is considered an offensive component in the immune system's lines of defense, taking its name from the ability to appropriately adapt in response to a variety of threats.

Acquired immunity encompasses numerous individual factors. Immunizations and antibodies created after previous exposure to a pathogen fall under acquired immunity. Considered both an offensive and a defensive component of the human immune system, certain types of acquired immunity are temporary or passive. For example, newborn babies borrow immunity from their mothers, first through the placenta during pregnancy and after birth through colostrum in the mother's breast milk. Later, as the child matures and experiences exposure to certain pathogens, the adaptive immune system assists the acquired immune system by creating memory cells or antibodies. This allows the acquired immune system to establish permanent defenses against certain types of threats.