Please match the complement pathways with the statements that most accurately describe them to test your understanding of the complement system, its origins, and basic functions. 1. Most specific, activated by antibody bound to microorganisms
2. Nonspecific reaction of a host serum protein that binds a sugar, called mannan, in microbial cell walls
3. Begins when complement proteins bind to normal cell wall and surface components of microbes

1. Most specific, activated by antibody bound to microorganisms - Classic complement pathway.

2. Nonspecific reaction of a host serum protein that binds a sugar, called mannan, in microbial cell walls - Lectin pathway.

3. Begins when complement proteins bind to normal cell wall and surface components of microbes - Alternative complement pathway.

Explanation:

The complement system is a protein cascade that is of key importance for the innate immune system to promote inflammation and thus counter an occurring infection.

The complement system can be activated in three different ways, by three different pathways.

When an unknown pathogen enters the body by breaking the first line of defense that is given by the epithelium, the first pathway to be activated is the alternative complement pathway because one of its proteins, C3b can bind to the surface of any cell and activate the cascade when the proper signal to do so is there (our cells have specific substances that C3b can recognize as non-harmful, and therefore it prevents the cascade to be activated in the absence of pathogens).

When the pathogen causing the infection has previously encountered the immune system, it is more likely that there are already antibodies for this specific pathogen in the infected tissues, thanks to the memory cells that are produced during the adaptative immune response. These antibodies will bind to the C1-complex and trigger the classical pathway. The antibodies will opsonize the pathogen.

The third pathway that can be activated is the lectin pathway. For this one to happen, there has to be a production of mannose-binding lectin by the liver that will occur between 24 to 48 hours of the beginning of the infection. This lectin will bind to the mannose residues on the surface of the pathogen and opsonize it.

When activated, the complement system induces the recruitment of phagocytes such as neutrophils and macrophages (inflammation) to phagocyte the opsonized microorganism, and will also rupture the membrane wall.

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