The adaptive immune response must be activated by cells of the innate immune system. These cells must recognized viruses and living cells like bacteria, fungi, and protozoans like amoebas. There are often common structural features in these different classes of cells. The cells of the innate system (dendritic cells, macrophages, eosinophils, etc) have receptors (Toll-like Receptors 1-10 or TLRs) that recognize the common pathogen associated molecular patterns (PAMPs) , which leads to binding, engulfment, signal transduction, maturation (differentiation), antigen presentation, and cytokine/chemokine release from these cells. Take for example dendritic cells, which reside in the peripheral tissues and act as sentinels. They can bind PAMPs which include:
- CHO/Lipids on bacteria surface (LPS)
- mannose (CHO found in abundance on bacteria,
- yeast dsRNA (from viruses)
- nonmethylated CpG motiffs in bacterial DNA
Bacterial and viral nucleic acids are recognized by intracellular TLRs in the cell after the they been taken up into the cells by endocytosis. Dendritic cells phagocytize microbial and host cells killed through programmed cell death (apoptosis). In the process of maturation, surface protein expression is altered, allowing the cells to leave the peripheral tissue and migrate to the lymph nodes where they activate T cells through the antigen presentation methods described above. They also control lymphocyte movement through release of chemokines. The very large figure below shows the processes involved in recognition of PAMPs by TLRs of innate immune system cells.