Acquired immunity, also called adaptive immunity, is a specialized defense system the body develops throughout a lifetime by learning to target specific pathogens it has encountered before. This contrasts with innate immunity, the general protection everyone is born with that acts as the body’s first line of defense. When these initial defenses are bypassed, the acquired immune system is activated, providing a more durable form of protection.
The Two Pathways to Acquiring Immunity
Acquired immunity develops through two distinct pathways: active and passive. Active immunity is established when an individual’s own immune system produces antibodies and specialized memory cells after being exposed to a foreign substance, known as an antigen. This process is self-generated and results in long-lasting protection because the body “remembers” the antigen.
The second pathway is passive immunity, which involves the transfer of pre-made antibodies from an external source. The individual’s body does not produce its own antibodies or memory cells, instead receiving them ready-made for immediate but temporary protection. This borrowed immunity’s effects diminish over a few weeks or months as the donated antibodies are naturally broken down and cleared from the system.
Developing Active Immunity
The body can develop active immunity through two primary methods: natural infection and vaccination. Natural active immunity is acquired after being exposed to a pathogen, experiencing the subsequent illness, and recovering. A common example is the immunity that develops after a person contracts and recovers from chickenpox, which typically provides lifelong protection against that specific virus.
Artificial active immunity is achieved through vaccination. This process introduces a harmless component of a pathogen, such as a weakened virus or a specific protein, into the body. This exposure stimulates the immune system to produce antibodies and memory cells without causing the actual disease. The body’s response to a vaccine creates a defense prepared for future encounters, allowing for immunity without the risks of the illness itself.
Receiving Passive Immunity
Passive immunity provides immediate, though short-term, protection by transferring ready-made antibodies to an individual. This happens naturally from a mother to her child. During pregnancy, Immunoglobulin G (IgG) antibodies are passed across the placenta to the fetus. After birth, Immunoglobulin A (IgA) antibodies are transferred through breast milk, particularly colostrum, shielding the newborn from infections.
This form of immunity can also be administered artificially through medical interventions. These treatments involve injecting antibody-containing preparations, known as antisera or immune globulins, sourced from humans or animals. This method is used when immediate protection is needed, such as after exposure to rabies or for individuals with weakened immune systems. Monoclonal antibody therapies are a modern example, providing a concentrated dose of specific antibodies to fight off an infection.
The Cellular Basis of Immunological Memory
The long-term protection from active immunity is rooted in specialized white blood cells called lymphocytes, specifically B cells and T cells. When the body first encounters a pathogen, B and T cells that recognize the specific antigen are activated. B cells produce proteins called antibodies, which can neutralize toxins or tag pathogens for destruction. T cells have multiple roles; helper T cells assist in activating B cells, while cytotoxic T cells directly kill infected host cells.
After the initial threat is neutralized, a small subset of these activated lymphocytes persists as memory B cells and memory T cells. These long-lived cells circulate throughout the body, sometimes for decades. If the same pathogen enters the body again, these memory cells recognize it immediately. This recognition triggers a secondary immune response that is much faster and more effective than the primary response, often preventing illness entirely.