Hurray for complement! Has this ever happened to you: you're thirty minutes into the third lecture your prof has mentioned the complement system and it's only then that you realize you don't actually know what complement is? Well it's a good time to figure out exactly what all complement is capable of doing, since these proteins are bound to come up in more than one lecture. The functions of complement are below, written (with one exception) in compliments :D If you are interested in the complement pathways, see the story on how rioting glue bottles caused incredible cellular damage.
Functions of Complement
Coating pathogens to make them better targets for attack by macrophages. Complement components involved here? C3b and C4b.
Yes, this would be the MAC attack. Works better for gram negative than gram positive bacteria. Why? Time to pull out those critical thinking skills.
Inflammatory Response Activation
You know those little pieces that break off during the pathways? Like C3a and C5a? Those actually don't just disappear off the face of the earth when they get the upswing-you're-out-of-the-picture-now arrow in the diagrams outlining the complement pathways. Instead, they get miffed at being not important enough for the main show and go and wreak havoc on basophils and mast cells, as well as smooth muscles and blood vessels. In other words, they contribute to inflammation.
You want to keep the wolves away from the door. Immunoclearance is important and problems in this area are related to Systemic lupus erythematosus, hence the wolf (lupus). C3b facilitates this one.
Remember that antibodies and complement can work together! I like to think of this as the principal and the vice principal walking a bad, bad student to the office for a talking to. There are several ways that viruses (and students!) can evade this dynamic duo but that's a story for another day.
Kindt, T., Goldsby, R., & Osborne, B. (2007). Kuby Immunology (6th ed., Chapter 7). New York: W. H. Freeman and Company.