Alternative pathway of the complement system


The complement system is a part of the innate immune system that helps to eliminate pathogens and promote inflammation. It consists of a series of proteins that circulate in the blood and can be activated by different triggers. The complement system has three main pathways of activation: the classical pathway, the lectin pathway, and the alternative pathway.

The alternative pathway is unique among the complement pathways because it does not require antibodies or other specific recognition molecules to initiate. Instead, it relies on the spontaneous activation of a key protein called C3, which can bind to foreign surfaces such as bacteria, viruses, or parasites. The alternative pathway can also amplify the activation of the other complement pathways by generating more C3 molecules.

The alternative pathway is important for host defense against various infections, especially those caused by encapsulated bacteria that can evade antibody responses. It also plays a role in inflammation, tissue repair, and immune regulation. However, excessive or uncontrolled activation of the alternative pathway can cause damage to host cells and tissues, leading to diseases such as atypical hemolytic uremic syndrome (aHUS), age-related macular degeneration (AMD), and systemic lupus erythematosus (SLE).

In this article, we will explore the steps and mechanism of the alternative pathway, the three distinct pathways to initiate it, the regulators that control it, and the applications and significance of it in health and disease.