Abstract

Antigen recognition and communication among cells (T, B and natural killer) of the immune system are regulated at the level of cell-surface antigen and transmembrane receptors. The extracellular regions of these receptors provide the immune specificity, whereas the intracellular region interacts with cytoplasmatic signaling molecules that are critical for the activation and termination of signal transduction pathways. The initial signaling events following the engagement of antigen receptors are regulated by the phosphorylation of characteristic signaling motifs that coordinate the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases) and, in such a way, regulate the activation or inhibition of the intracellular response. Some receptors like CD154, CD28, CD3 transmit mainly positive co-stimulatory signals, whereas others - such as CTLA-4, killer inhibitory receptors (KIRs) or leukocyte immunoglobulin-like receptors (LIRs) - function principally as inhibitory receptors.

In addition, a group of dual-function receptors - such as the CD150 subfamily within the CD2 family - can act as either positive or negative regulators.