Fig 1. Principle mechanisms of innate and adaptive immune responses

Innate Immune System

After a pathogen has infiltrated barriers of entry such as the skin, the host innate immune system attempts to prevent infection from the foreign invaders. One of the primary innate defenses consists of complement activation. Complement is a group of soluble serum proteins mainly produced by the liver. These complement components become activated during an infection through the classical, lectin, or alternative pathway. In all three of these pathways, proteins (C1, lectin, C3) bind either antibodies or pathogen antigens to induce the complement cascade. This cascade and its terminal complement proteins can untie to form membrane attack complexes, opsonize pathogens to facilitate phagocytosis, and promote an inflammatory response which includes vasodilation and increasing vascular permeability.

The innate immune system also relies on the association of complements with leukocytes which are various white blood cells. The innate leukocytes include: neutrophils, eosinophils, basophils, mast cells, monocytes/macrophages, and dendritic cells.

1) Neutrophils are produced in large quantities in the early stages and migrate quickly to the infection site to phagocytize, and kill pathogens with enzymes and toxic granules.

2) Eosinophils are present in relatively low quantities in blood and have surface receptors for antibodies and C3b that have already binded pathogens. The eosinophils recognize these markers and release cationic proteins to create pores in the pathogen that allow granules, enzymes, lipase, peroxidase, etc. to enter the cell and destroy it.

3) Basophils are also present in relatively low concentration in the blood. They bind antibodies, degranulates, and release toxics. They also release histamine when basophils bind active C3a and C5a fragments.

4) Mast cells differentiate in tissue and are involved in the protection of internal surfaces and also provide histamine for inflammation.

5) Monocytes are phagocytes in the blood that migrate to tissues to differentiate into macrophages. Macrophages phagocytize invading cells and also promote inflammation. Macrophages can also activate adaptive lymphocytes and cleans up after adaptive responses.

6) Dendritic cells are antigen presenting cells that phagocytize pathogens and displays specific antigens on its surface. They are involved in activating T lymphocytes and provide the transition from innate to adaptive response.