**4. Immune receptors and their function**

Cells of the immune system intercommunicate by ligand-receptor interactions between cells and/or via secreted molecules called cytokines. Cytokines produced by lymphocytes are termed lymphokines (i.e., interleukins & interferon-γ), and those produced by monocytes and macrophages are termed monokines [89]. The


**89**

**Figure 2.**

*Diagram showing CD markers on various immune cell types.*

**Figure 1.**

*Immune Cell Activation: Stimulation, Costimulation, and Regulation of Cellular Activation*

main receptors in the immune system are pattern recognition receptors (PRRs), Toll-like receptors (TLRs), killer activated and killer inhibitor receptors (KARs and KIRs), complement receptors, Fc receptors, and B & T cell receptors. Many are

*DOI: http://dx.doi.org/10.5772/intechopen.81568*

*Principal surface markers of lymphocyte populations.*

## **Table 2.**

*Major features and functions of mononuclear leukocytes.*

*Immune Cell Activation: Stimulation, Costimulation, and Regulation of Cellular Activation DOI: http://dx.doi.org/10.5772/intechopen.81568*

main receptors in the immune system are pattern recognition receptors (PRRs), Toll-like receptors (TLRs), killer activated and killer inhibitor receptors (KARs and KIRs), complement receptors, Fc receptors, and B & T cell receptors. Many are

**Figure 1.**

*Immune Response Activation and Immunomodulation*

**4. Immune receptors and their function**

Receptor TcR IgM &

CD8

Other surface markers CD3, CD4,

*Major features and functions of mononuclear leukocytes.*

Mononuclear cells in blood

**3.7 Receptor dimerization and immune cell stimulation and functioning**

Chemokines are chemotactic cytokines that mediate their effects on leukocytes through a number of G protein-coupled, seven transmembrane-spanning (STM) receptors [84]. Specificity is provided by patterns of receptor and G protein expression, ligand potency, and levels of receptor desensitization. Interactions among receptors are mediated through a process known as receptor cross regulation, or heterologous desensitization [85]. Ali et al*.* [86] showed that thrombin receptor activation causes phosphorylation of several chemoattractant receptors, including the IL-8 receptor CXCR1, the C5a receptor, and the receptor for platelet-activating factor 3. Ben-Baruch et al*.* [87] have shown that homologous desensitization

through phosphorylation of the IL-8 receptor CXCR2 occurs in response to its native ligands IL-8 and neutrophil-activating peptide (NAP)-2. Grimm et al*.* [28] showed that opiates, acting through δ and μ subclasses of opioid receptors expressed on human monocytes and neutrophils, are capable of inhibiting subsequent migratory responses to chemokines, and that this process of heterologous desensitization or trans-deactivation is associated with phosphorylation of chemokine receptors. Szabo et al*.* [88] showed that the chemotactic activities of both μ- and δ-OR are desensitized following activation of the chemokine receptors CCR5, CCR2, CCR7, and CXCR4 but not of CXCR1 or CXCR2 receptors. The inhibition of CCL3 and CCL5 responses following opioid pretreatment is consistent with the desensitization of either CCR1 or CCR5 or both. This receptor cross talk results in heterologous desensitization and phosphorylation of some of the chemokine receptors, which subsequently contribute to the immunosuppressive effects of the opioids.

Cells of the immune system intercommunicate by ligand-receptor interactions between cells and/or via secreted molecules called cytokines. Cytokines produced by lymphocytes are termed lymphokines (i.e., interleukins & interferon-γ), and those produced by monocytes and macrophages are termed monokines [89]. The

Antigen recognition + + − − Antigen presentation − + − + Antibody production − + − − Cellular immunity + − − + Immune regulation + − + + Phagocytosis − − − + Cytotoxicity + − + +

IgD

**T cells B cells NK cells Monocytes/**

**macrophages**

CD16 CD11b

CD19–21 CD56 CD14

~75 ~10 10 5

**88**

(%)

**Table 2.**

*Principal surface markers of lymphocyte populations.*

**Figure 2.** *Diagram showing CD markers on various immune cell types.*

phagocytic receptors that stimulate ingestion of the pathogens they recognize. Some are chemotactic receptors, such as the f-Met-Leu-Phe receptor, which binds the N-formylated peptides produced by bacteria and guide neutrophils to sites of infection. A third function, which may be mediated by some of the phagocytic receptors as well as by specialized signaling receptors, is to induce effector molecules that contribute to induce innate immune responses and molecules that influence the initiation and nature of any subsequent adaptive immune response [1]. Various immune cell functions regulated by receptors on immunocytes are summarized in **Table 3** and **Figure 3**.
