4.1 Receptors for PAMPs and DAMPs present in cytosol

In addition to membrane bound TLR many receptors are present in the cytosol which can sense the invading antigenic structures.

Nucleotide oligomerization domain (NOD) like receptors present in the cytosol is specialized to sense PAMP and DAMP and recruit the chemical mediators of inflammation. NOD1 can sense invading gram negative bacteria while NOD2 can recognize the muramyl dipeptide from both gram positive and negative bacteria.

Retinoic acid-inducible gene-I (RIG-I) like receptors are another sensors present in the cytosol which are specific against viral RNA and induce the production of type I interferon.

Carbohydrates present on the surface of microbes are recognized by C-type lectin receptors and facilitate their phagocytosis. Similarly, mannose receptors are also playing an important role in sensing the carbohydrate entity present over the surface of many microorganisms. Another group of receptor called dendritic cell-associated C type lectin-1 (Dectin-1) and dectin-2 are important sensors of fungal antigens.

In addition, many cell types express different receptors involved in the phagocytosis of the antigens. Scavenger receptors present over the surface of macrophages senses specifically the oxidized lipoproteins from bacterial cell. N-formyl met-leu-phe receptors are expressed over the surface of neutrophils and macrophages and are involved in the recognition of the N formylmethionyl residues of bacterial origin.

4.2 Cellular components of innate immune system

Many different cellular components are involved in the proper functioning of the body innate immune system.

4.2.1 Epithelial cells

Epithelial cells over the skin surface are the physical barriers for the invading pathogens. Epithelial cells produce antimicrobial substances such as defensins and cathelicidins which also hinder the entry of pathogens. Epithelial barriers include skin, gastrointestinal and respiratory mucosa. Intraepithelial T lymphocytes present in the skin and gastrointestinal tract can respond to the encountering pathogens.

4.2.2 Phagocytes

Macrophages and neutrophils are the first line of defense against the pathogens and are specialized in phagocytic function. Usually phagocytic cells are involved in killing of the microbes and secretion of cytokines that mediate the inflammatory response.

4.2.3 Dendritic cells

They are one of the most important components of the innate immune system. Their role of presenting the antigens to the cells of adaptive immune system makes it unique among the others. They express variety of TLRs, PAMPs and DAMPs for the recognition of the pathogens and present it to the naïve T lymphocytes to trigger the adaptive immune response.