Principals of innate and adaptive immunity. Immunity to microbes & fundamental concepts in immunology

Microorganisms such as bacteria that penetrate the epithelial surfaces of the body for the first time are met immediately by cells and molecules that can mount an innate immune response. Phagocytic macrophages conduct the defense against bacteria by means of surface receptors that are able to recognize and bind common constituents of many bacterial surfaces. Bacterial molecules binding to these receptors trigger the macrophage to engulf the bacterium and also induce the secretion of biologically active molecules. Activated macrophages secrete cytokines, which are defined as proteins released by cells that affect the behavior of other cells that bear receptors for them. They also release proteins known as chemokines that attract cells with chemokine receptors such as neutrophils and monocytes from the bloodstream. Macrophages in response to bacterial constituents initiate the process known as inflammation. Antigen-presenting cells (APCs) are a heterogeneous group of immune cells that mediate the cellular immune response by processing and presenting antigens for recognition by certain lymphocytes such as T cells. Classical APCs include dendritic cells, macrophages, Langerhans cells and B cells. Innate lymphoid cells (ILCs) are immune cells that belong to the lymphoid lineage but do not express antigen-specific receptors. These cells have important functions in innate immune responses to infectious microorganisms and in the regulation of homeostasis and inflammation.