Humoral Factors of Innate Immune System

Lysozyme

Lysozyme is one of the major bactericidal agents in secretions and particularly helps to protect vulnerable sites such as the eyes and nasal passages. The lysoszyme exerts bactericidal effects by digesting bacterial cell walls.

The gram-positive bacteria are more sensitive to lysozyme action than gram-negative bacteria as the outer membrane of gram-negative bacteria helps to protect them.

Complement

The complement system is a group of about 30 proteins within the body fluids of all vertebrates and some invertebrates. The main functions of complement are to promote phagocytosis or causes lysis of an invading organism.

The complement acts as a cascade, like the blood clotting system. It begins with the the early enzymes in the cascade binding to invading bacteria and fungi as they have an affinity for components of microbial cell membranes. This binding initiates a cascade so that the binding of one molecule will eventually lead to the fixation of millions of later molecules. Some of the the early components act as targets for phagocytes while the later components punch holes in bacteria, causing their lysis.

Interferons

The lysozyme and complement have only marginal effects on virus infections because these are intracellular so the body has evolved non-specific mechanisms to protect against viruses with the most notable of these being the interferons.

Interferons are small polypeptides produced mainly by virus-infected cells. The main functions are of interferon is to interact with uninfected cells and render them resistant to infection by increasing the production of enzymes that digest viral nucleic acids. The interferon can then also cause an increase in MHC (Major Histocompatibility Complex) Class I and II expression on all cells, so if they are infected, the immune system can recognise the infection and take further steps of limiting viral replication. An example is the lysis of the infected cell by cytotoxic T lymphocytes, or the induction of apoptosis by Natural Killer Cells. It is for this reason, as well as being a chemoattractant and causing the further release of other cytokines, that interferon is a potent initiator of the immune response.

Originally funded by the RVC Jim Bee Award 2007