The adaptive immune system is one of the two components of a vertebrate's immune system. The other component, the innate immune system, triggers the adaptive system, which targets pathogens and attempts to destroy them. What makes the adaptive immune system so valuable is its ability to remember the first and recognize any subsequent infections caused by the same pathogen. Thus, the adaptive immune system is better equipped to fight it each time. This makes most vertebrates, humans especially, quite resilient and less likely to be harmed by a subsequent infection, if they survive the initial infection.
One component of the vertebrate dual immune system is the innate immune system. It is the more primitive of the two and is the body's first reaction to a pathogen, which is an organism that can produce disease. Unlike the adaptive immune system, the innate system consists of a series of automatic defenses that are not specific to a pathogen type.
If the innate immune system is unsuccessful in eradicating a pathogen, then the adaptive immune system is activated. The adaptive immune system is specially suited to detect harmful antigens, which are any substances, including those found on pathogens, that trigger an immunological response when they are recognized as foreign to the body. Viruses and bacteria will activate this process.
Lymphocytes make up the adaptive immune system's arsenal against harmful antigens. These white blood cells travel through the body in search of foreign or harmful antigens. B cells and T cells are the body's two approaches to countering harmful antigens.
B cells, also called B lymphocytes, are the humoral immune response (HIR) to antigens. This form of defense is characterized by the production of antibodies by the B cell. The antibodies, which are actually proteins called immunoglobins, are dispensed by the B cell after identifying a threat and attaching themselves to the invading cells. The antibodies mark the cells so that the innate immune system can target them, preventing pathogens and their toxins from attaching themselves to host cells and reproducing.
T cells conduct cell-mediated immune responses, which refer to the ability of cells to attack pathogens directly, without antibodies. Cells with foreign antigens are targeted by T cells and killed before the infection can take hold. They also trigger other defense mechanisms, such as natural killer cells (NKCs), which are found in the innate immune system. NKCs release proteins that cause death in the target cell.
Without an adaptive immune system, vertebrate life would undergo a much higher casualty rate from even the most common infections. When a lymphocyte detects a harmful antigen, its deoxyribonucleic acid (DNA), which gives a cell its defining characteristics, is permanently altered, meaning that the lymphocyte is now specialized in combating the specific pathogen.
The ability of cells to remember antigens is what makes vaccinations effective at preventing infection. Vaccinations activate the adaptive immune system. Lymphocytes discover the vaccine in the body and destroy it. The white blood cells now recognize the pathogen and become equipped to fight it.
The ability of the adaptive immune system to distinguish between the body and foreign harmful entities is key to proper immune function. If the lymphocytes mistake a part of the body or a helpful foreign substance as harmful, then an autoimmune disorder can develop. This causes the adaptive immune system to build up a defense against necessary or helpful substances and to destroy them. The skin condition eczema is a common form of autoimmune disorder.