Which one of the following complement components is NOT involved in the alternative pathway?

The correct answer indicates C1 is not involved in the alternative pathway of complement activation. The alternative pathway is part of the innate immune response and initiates complement activation in a manner that is different from the classical pathway.

In the alternative pathway, C3 plays a central role, as it is the component that gets activated spontaneously and leads to the cascade involved in this pathway. Unlike C1 and C2, which are specific to the classical pathway and require antibody-antigen complexes for activation, the alternative pathway can be triggered directly by foreign surfaces, such as bacteria or other pathogens.

C5 is also involved in both the classical and the alternative pathways, as it is part of the later steps of the complement activation where it gets cleaved to form C5a and C5b, leading to the formation of the membrane attack complex.

Therefore, since C1 and C2 are specific to the classical pathway while C3 and C5 are utilized in both pathways, it becomes clear that C1 is the complement component that does not play a role in the alternative pathway.