Introduction
Gap Junctions (GJs) are the most studied mechanism of cell-cell close contact communications. Though immune cells are generally sparse within tissues, they can ‘touch’ each other during normal immune response or in pathological conditions, such as leukemia, when lymphoid organs are tightly infiltrated by clonally identical proliferating cells. In this case they can contact through GJs.
Chemokines are a family of chemoattractant cytokines (small proteins secreted by cells that influence the immune system) which play a vital role in cell migration through venules from blood into tissue and vice versa, and in the induction of cell movement in response to a chemical (chemokine) gradient by a process known as chemotaxis (Figure 1).
In order to mount and coordinate an effective immune response, a mechanism by which lymphocytes, inflammatory cells and haematopoietic cells can communicate with each other is required. Cytokines perform this function. Cytokines are a large, diverse family of small proteins or glycoproteins (usually smaller than 30 kDa). Although initially described for their immunomodulatory capabilities, additional roles separate from the immune system in developmental processes are also documented, such as cell differentiation and directed migration.
Host defence peptides (HDPs) are also known as anti-microbial peptides, although more recently they have been shown to demonstrate a number of other immunomodulatory functions. They were first identified as having antibiotic effects on a wide range of bacteria, fungi and viruses. Most HDPs are cationic, and this is thought to mediate their anti-microbial action by interfering with microbial membranes.
Immunoglobulin A (IgA) is the first line of defence in the resistance against infection, via inhibiting bacterial and viral adhesion to epithelial cells and by neutralisation of bacterial toxins and virus, both extra- and intracellularly. IgA also eliminates pathogens or antigens via an IgA-mediated excretory pathway where binding to IgA is followed by polyimmunoglobulin receptor-mediated transport of immune complexes.
Upon a microbial infection, the body needs to be alerted to the presence of potential harmful pathogens. This is achieved through specialised receptors known as pattern recognition receptors (PPRs) which are predominantly expressed on immune cells.
