For most of us, developing a bacterial infection is at a minimum no fun, and usually quite irritating. We don’t really consider what actually goes on inside of our bodies, and as a result, fail to appreciate the vast population of cells and molecules dutifully combating the bacterial invaders. Maybe we ought to reconsider our sentiments towards being sick.
The immune response following each attack of a foreign entity (‘pathogen’) on our bodies is highly complex and actually pretty amazing. Two types of responses may occur – the innate and the adaptive immune responses. The innate response is the first line of defense, and is mitigated by physical barriers like the skin and mucosa lining our digestive and respiratory tracts, as well as by actions like sneezing and coughing. White blood cells (‘leukocytes’) carry out various functions like phagocytosis (engulfing and digesting a large pathogen into small parts) and target cell lysis (killing), which are examples of induced innate responses. If the innate response does not effectively clear our bodies of the pathogen, then certain immune cells (‘lymphocytes’) are subsequently activated to launch a highly specific and intense counterattack involving antibody secretion and precise killing of pathogen.
An important connection between the two types of immune responses is established by something called the complement system. Complement is composed of a collection of various serum (present in bodily fluid) proteins that become sequentially triggered in a closely controlled cascade. The chronological activation of each component protein in the complement system can be thought of as a medley relay race in swimming, where each swimmer swims a different stroke, and the first swimmer must finish his or her part of the race before the next swimmer can begin. However unlike a relay race, where the winners receive an award, complement activation results, for example, in the killing of a pathogen, or in the coating of a pathogen with a particular complement protein (called ‘opsonization’). Opsonization is important in helping leukocytes engulf and digest pathogens more easily, so that the foreign entity may be cleared from circulation and tissue with greater efficiency. Many of the complement proteins are named with the letter C followed by numbers 1-9. Other proteins are named as Factors that have been assigned a single letter (i.e. Factor B, Factor D, etc.). The complement system can be activated by three biochemical pathways: classical, lectin, and alternative. The classical pathway is dependent on the presence of antibody, which means that the adaptive response must be initiated before complement can be activated. However, the lectin and alternative pathways are purely part of innate immunity, and do not involve the interaction with antibody. This review will focus on lectin and alternative complement activation, as an article recently published elucidated a novel role of a protein interestingly involved in both of these pathways (1).
