Recover from measles and you are protected for life. Recover from chickenpox and you may not be. The gap between those two outcomes comes down to how the body’s two layers of defense work, and why only one of them keeps a record.
The fast layer
The innate immune system is the older of the two. Plants, fungi, insects and vertebrates all have it, and for most of them it is the main protection they get. It does not aim at a particular germ. It reacts to broad categories of invader, and it does not leave lasting immunity behind.
Much of it is simple barrier work. Leaves have a waxy coating, insects have an exoskeleton, and most vertebrates have skin. Coughing and sneezing push irritants out of the airways. Saliva and tears fight bacteria, and stomach acid and proteases kill what gets swallowed.
When tissue is injured or infected, cells release signaling molecules called cytokines and eicosanoids. These raise body temperature, widen blood vessels and call in more immune cells. That is inflammation, and the redness, swelling, heat and pain that come with it are the visible result.
The patrol is staffed by white blood cells. Macrophages, neutrophils and dendritic cells engulf pathogens and dead cells. Mast cells and basophils manage allergic inflammation, eosinophils go after multicellular parasites, and natural killer cells destroy compromised host cells and cancer cells.
The layer that remembers
The adaptive system evolved in early vertebrates and is found only in them. It is slower. In humans it takes 4 to 7 days to mount a significant response. What it offers in return is precision: it is highly specific to each pathogen the body has met.
Two kinds of lymphocyte do the work. B cells make antibodies, proteins also known as immunoglobulins, which travel through the blood and bind to a foreign antigen so it cannot attach to the host. T cells handle the cell-mediated side. They develop in the thymus, after leaving the bone marrow, where both cell types are produced. One group, the cytotoxic T cells, kills cells infected with viruses.
The scale is large. The body holds about 2 trillion lymphocytes, 20 to 40 percent of its white blood cells, with a combined mass close to that of the brain or liver. Only 2 percent are in the bloodstream at any moment. The other 98 percent move through tissues and the lymphatic system, including the lymph nodes and spleen, and each hour 1 to 2 percent of the pool recirculates to improve the odds of meeting a matching pathogen.
After an infection, memory B cells and memory T cells remain. They are the survivors of past fights, and they are why a second encounter brings a faster, stronger response. Vaccination is built on the same process.
The system can also misfire. Sometimes it fails to tell harmful foreign molecules from harmless ones, and the result is hayfever, asthma or another allergy.
Where the two meet
The layers do not work in isolation. The adaptive response starts when a pathogen slips past the innate defenses and produces enough antigen, along with “stranger” or “danger” signals that activate dendritic cells. A dendritic cell engulfs the invader, travels to a lymph node, chops the pathogen into pieces and shows those fragments to passing T cells. That handoff is what wakes the second layer.
The word “adaptive” is younger than the science. Robert Good first used it in 1964, writing about antibody responses in frogs, and it did not become common until the 1990s.
Sources
- Adaptive immune system – Wikipedia
- Innate and Adaptive: Two Layers of Immune System Defense – Profolus
