That’s a complicated title. What it means is that complement is part of out innate immune response, i.e. it is pre-made, ready-to-go and does not adapt over time in response to immune challenges. However, it is guided by antibodies, which are part of the adaptive immune response.
What complement is, is a number of proteins that come together and activate one another in a cascade that coordinates the formation of a hole- or pore- through the membrane of foreign cells. The cascade begins when the first of the complement proteins associates with antibodies that are opsonizing a foreign cell.
Once recruited, complement proteins will activate in a cascade (see movie) in which small parts of the proteins break off and act as anaphylatoxins recruiting and activating immune cells to the region. Meanwhile, the larger protein elements will assemble into pore-forming complexes that will kill the invading cell.
With this in mind, watch the animation below looking for:
1) antibodies binding to foreign cells
2) early complement proteins being recruited
3) complement breaking into large and small proteins
4) the smaller ones floating away to recruit immune cells
5) the larger ones forming membrane pores and killing the invading cell.
(don’t worry about the sequence of events or the specific proteins involved)