<Tr> <Td> Blood - brain barrier </Td> <Td> endothelial cells (via passive diffusion / osmosis & active selection). P - glycoprotein (mechanism by which active transportation is mediated) </Td> </Tr> <P> Anatomical barriers include physical, chemical and biological barriers . The epithelial surfaces form a physical barrier that is impermeable to most infectious agents, acting as the first line of defense against invading organisms . Desquamation (shedding) of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces . Lack of blood vessels and inability of the epidermis to retain moisture, presence of sebaceous glands in the dermis provides an environment unsuitable for the survival of microbes . In the gastrointestinal and respiratory tract, movement due to peristalsis or cilia, respectively, helps remove infectious agents . Also, mucus traps infectious agents . The gut flora can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces . The flushing action of tears and saliva helps prevent infection of the eyes and mouth . </P> <P> Inflammation is one of the first responses of the immune system to infection or irritation . Inflammation is stimulated by chemical factors released by injured cells and serves to establish a physical barrier against the spread of infection, and to promote healing of any damaged tissue following the clearance of pathogens . </P> <P> The process of acute inflammation is initiated by cells already present in all tissues, mainly resident macrophages, dendritic cells, histiocytes, Kupffer cells, and mast cells . These cells present receptors contained on the surface or within the cell, named pattern recognition receptors (PRRs), which recognize molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen - associated molecular patterns (PAMPs). At the onset of an infection, burn, or other injuries, these cells undergo activation (one of their PRRs recognizes a PAMP) and release inflammatory mediators responsible for the clinical signs of inflammation . </P>

Which two types of leukocytes (white blood cells) play a main role in the innate immune system