4.9 Antibodies and B cells
Antibodies are very large proteins that contribute to adaptive immunity. There are several types, but the most abundant antibody molecules in humans each contain about 25 000 atoms. A distinguishing feature of antibodies is that their structure includes at least two binding sites for antigens. Most antibodies resemble the simplified representation in Figure 10.
B cells produce antibodies and also use them as their antigen receptors. The B cells carry antibodies embedded by the ‘tail’ in their outer cell membrane, with the binding sites facing outwards (Figure 10). This enables the B cell to bind to antigens that fit the binding sites in the antibodies it carries on its surface. This binding event is essential (but not sufficient on its own) to activate B cells into making a lot more antibody molecules that recognise the same antigen. These antibodies are released by the B cells and circulate in the bloodstream, tissue fluids and the lymphatic system. Antibodies are also abundant in the mucus membranes lining the respiratory system, the gut and the reproductive system, i.e. the sites in the body in contact with substances such as air, food, drinking water and sexual fluids that could contain pathogens.
Antibodies are often portrayed in the media as if they were ‘magic bullets’ that attack pathogens, but in fact they are more like ‘waving flags’ with a message that reads ‘here is a pathogen – come and destroy it’. When antibodies bind to a pathogen, they simply label it for destruction by leukocytes with the innate ability to phagocytose (engulf) it, or cytotoxic (cell-killing) leukocytes and defensive proteins. You can think of them as recruiting the cells and defensive proteins of the innate immune system to join the attack.
We conclude this tour of adaptive immunity by describing the T cells.