depends upon the information it receives from the innate phase. The second phase is

triggered when the information gathered by the innate system is conveyed to the

adaptive immune system, which in turn, uses this information to mount a response

specific to the pathogen. It takes longer than the innate immune system, but the

adaptive system makes up in specificity what it lacks in the rapidity of response.

3.2

INNATE IMMUNE SYSTEM

3.2.1

PATTERN RECOGNITION RECEPTORS

The innate immune system recognizes molecular patterns associated with patho-

gens. Such pathogen-associated molecular patterns or PAMPs include various li-

popolysaccharide molecules on bacterial surfaces, yeast cell wall components, and

structures such as flagella and microbial nucleic acids. The cells of the innate im-

mune system possess special receptors called pattern recognition receptors (PRRs)

that can distinguish these general molecular patterns, thereby detecting the presence

of foreign organisms in the environment [1]. One such family of receptors is named

toll-like receptors (TLR), referring to “toll,” a drosophila receptor, that was the first

member of this family to be identified. As shown in Figure 3.2, TLRs 2, 4, 6, and 10

FIGURE 3.2 The human toll-like receptor family. This figure provides examples of the

microbes and their components that are recognized by the members of the human toll-like

receptor (TLR) family. Receptors localized to the plasma membrane, TLRs 1, 2, and 6 re-

cognize various components of microbial cell membrane and cell walls such as lipopoly-

saccharides, peptidoglycans and zymosan. TLR5 recognizes bacterial flagellin. Intracellular

TLRs 3, 7, 8, and 9 are present on the endolysosomal membrane and recognize bacterial and

viral nucleic acids.

Introduction to basic immunology

37