第42章免疫系

Antibodies synthesized by the mucosal immune system include IgA and IgM. Activated B cells differentiate into mucosal plasma cells that synthesize and secrete dimeric IgA, and to a lesser extent, pentameric IgM. Secreted IgA is abundant in tears, saliva, breast milk, and in secretions of the gastrointestinal and respiratory tracts. Antibody secretion results in a local humoral response at epithelial surfaces and prevents infection of the mucosa by binding and neutralizing pathogens.

The variety, specificity, and reliability of antibodies makes them ideally suited for certain medical tests and investigations. Radioimmunoassays (RIA), for example, rely on the antigen-antibody interaction to detect the presence and/or concentration of certain antigens. Developed by Rosalyn Sussman Yalow and Solomon Berson in the 1950s, the technique is known for extreme sensitivity, meaning that it can detect and measure very small quantities of a substance. It is used in narcotics detection, blood bank screening, early cancer screening, hormone measurement, and allergy diagnosis. Based on her significant contribution to medicine, Yalow received a Nobel Prize, making her the second woman to be awarded the prize for medicine.

The basic principle of radioimmunoassay is competitive binding, where a radioactive antigen competes with a non-radioactive antigen for a fixed number of antibody or receptor binding sites. First, an antigen is made radioactive (tagged), and then it is mixed with a known quantity of its specific antibody. When a sample. such as one from the patient's blood, is introduced, antibodies will cease binding to the tagged antigen and instead bind to the untagged one, a process called displacement. After several steps, the amount of newly unbound tagged antigen is measured to see how much displacement occurred, which indicates the presence and concentration of the antigen in the blood.

For example, to measure insulin levels, the first step is to mix known amounts of radioisotope-tagged insulin and antibodies. These combine chemically. Next, a small amount of the patient's blood is added. The insulin contained in the blood displaces some of the tagged insulin. The tagged insulin is then measured with isotope detectors, and the patient's insulin level is calculated.

Antibodies (immunoglobulins) are the molecules secreted from plasma cells that mediate the humoral immune response. There are five antibody classes; an antibody's class determines its mechanism of action and production site but does not control its binding specificity. Antibodies bind antigens via variable domains and can either neutralize pathogens or mark them for phagocytosis or activate the complement cascade.