Immunology & Inflammation Antibodies

Immune cells possess cell surface markers designated as CD (cluster of differentiation) molecules. These cell surface proteins serve as cell surface markers for identification of the diverse immune cell types, but also have roles as cell surface receptors or ligands involved in antigen presentation, signaling and cell adhesion. There are more than three hundred cell surface markers identified in human immune cells, as well as other nonimmune cells, such as endothelial and epithelial cells, which often express these markers.

Cytokines are small proteins secreted by immune cells to communicate and alter the actions of other immune cells. They can be pro-inflammatory or anti-inflammatory, thus promoting or inhibiting the proliferation and functions of other immune cells. Cytokines are often called by other names depending mostly on the cell that produces them or on their biological functions. Lymphokines are produced by lymphocytes, monokines are released by monocytes, and interleukins are produced in one leukocyte but act on other leukocytes. Chemokines have chemotactic roles, which include the activation and migration of leukocytes. Different immune cells can secrete the same cytokine, while the same cytokine can act on several immune cells. Cytokines can act in an autocrine, paracrine or endocrine manner, and often the secretion of one cytokine promotes secretion of additional cytokines in the other immune cells.

Other proteins besides cell surface immune markers and cytokines also participate in immune responses. For instance, the complement system is involved in the defense against infections and is mediated by a group of plasma proteins that interact with each other on the surface of a pathogen. It complements the defense responses mediated by an antibody, but can also act in the absence of antibody-mediated reactions. Three pathways can cause complement activation, the classical pathway, the lectin pathway and the alternative pathway. Complement activation leads to opsonization of pathogens, recruitment of inflammatory cells, and direct killing of pathogens.