are present on the cell surface and recognize extracellular pathogens. Others, such

as TLR 7, 8, and 9 are present on the surface of endosomes and can recognize

nucleic acids from intracellular pathogens [2]. Other families of pattern recognition

receptors include C-type lectin receptors (CLRs), NOD-like receptors (NLRs), and

RIG-like receptors (RLRs) that detect the presence of molecules derived from

pathogens in the cytoplasm [1],

3.2.2

CYTOKINES AND CHEMOKINES

As shown in Figure 3.3, upon recognition of a foreign organism, cells of the

innate immune system secrete small molecules called cytokines to send a danger

signal to the other elements of the immune system. These molecules are re-

sponsible for the communication between different cell types, stimulating certain

functions, repressing others, thereby orchestrating a coordinated response to the

external threat. A subset of cytokines called chemokines can induce chemotactic

activity, recruiting various cell types to the site of infection. The recruited cell

type also secretes cytokines and chemokines, thereby amplifying the response [3].

It is important to note that, in addition to the cells of the innate immune system,

other cell types such as epithelial cells, endothelial cells, and fibroblasts also

express PRRs and thus can also contribute to the inflammatory response by

producing cytokines.

3.2.3

CELLS OF THE INNATE IMMUNE SYSTEM

As shown in Figure 3.1, there are different types of cells that make up the innate

immune system. One important category is made up of phagocytes.

FIGURE 3.3 Cytokines. Microbes, damaged or infected cells, are recognized by, and serve,

as a stimulus for healthy cells to induce small signalling molecules known as cytokines.

Cytokine-binding to receptors on immune cells convey a signal that triggers a response in the

form of a biological activity.

38

Bioprocessing of Viral Vaccines