The immune system is a collection of mechanisms within the body designed by nature to defend the body against pathogens, tumor cells and other foreign material.
There are two layers of defense systems. The innate or natural immune system which provides an immediate but non-specific response to foreign materials and the adaptive or acquired immune system which recognizes and retains specific responses in its memory, thereby enabling the host to mount faster and stronger onslaught on foreign pathogens.
Your question refers to the innate immune system. Humoral factors generally play a key role in the body's response to invasion by foreign pathogens and they can be found in serum formed at the site of an infection.
The main humoral factor which the innate immune system uses is the complement system. The complement system is activated by three biochemical pathways:
- The classical complement pathway which is triggered by the complement protein binding to specific antibodies.
- The alternative complement pathway which is triggered by the complement protein binding directly to the surface of the antigen
- The mannose-binding lectin pathway which is similar to the classical pathway but which involves a protein that binds to mannose residues on multiple pathogens.
The outcomes of the three complement pathways are the coating of the surface of pathogens marking them for destruction, production of peptides that recruit inflammatory cells and the disruption of the plasma membrane barrier directly killing the cells.