<Li> Macrophages that destroy foreign material, such as bacteria . </Li> <P> Type 1 pneumocytes (or membranous pneumocytes) form the structure of the alveolus and are responsible for the gas exchange in the alveolus . Type 1 pneumocytes are squamous epithelial cells which are characterized by a superficial layer consisting of large, thin, scale - like cells; they also cover 95% of the alveolar surface, although they are only half as numerous as Type 2 pneumocytes . Type 2 pneumocytes are important in that they can proliferate and differentiate into type 1 pneumocytes, which cannot replicate and are susceptible to a vast numbers of toxic insults . Type 2 pneumocytes are also important because they secrete pulmonary surfactant (PS), which consists 80--90% of phospholipids ((phosophatidylcholine (PC), phosphatidyglycerol (PG), phosphaditylinositol (PI)) and 5 - 10% of surfactant proteins (SP - A, SP - B, SP - C, AND SP - D). PS is synthesized as lamellar bodies, which are structures consisting of closely packed bilayers that are secreted and then undergo transformation into a morphological form called tubular myelin . PS plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension . By lowering alveolar surface tension, PS reduces the energy required to inflate the lungs, and reduces the likelihood of alveolar collapse during expiration . Loosely attached to these alveoli sacs are the alveolar macrophages that protect the lungs from a broad array of microbes and aerosols by devouring and ingesting them through phagocytosis . </P> <P> Alveolar macrophages are phagocytes that play a critical role in homeostasis, host defense, the response to foreign substances, and tissue remodeling . Since alveolar macrophages are pivotal regulators of local immunological homeostasis, their population density is decisive for the many processes of immunity in the lungs . They are highly adaptive components of the innate immune system and can be specifically modified to whatever functions needed depending on their state of differentiation and micro-environmental factors encountered . Alveolar macrophages release numerous secretory products and interact with other cells and molecules through the expression of several surface receptors . Alveolar macrophages are also involved in the phagocytosis of apoptotic and necrotic cells that have undergone cell - death . They must be selective of the material that is phagocytized because normal cells and structures of the body must not be compromised . To combat infection, the phagocytes of the innate immune system facilitates many pattern recognition receptors (PRR) to help recognize pathogen - associated molecular patterns (PAMPs) on the surface of pathogenic microorganisms . PAMPs all have the common features of being unique to a group of pathogens but invariant in their basic structure; and are essential for pathogenicity (ability of an organism to produce an infectious disease in another organism). Proteins involved in microbial pattern recognition include mannose receptor, complement receptors, DC - SIGN, Toll - like receptors (TLRs), the scavenger receptor, CD14, and Mac - 1 . PRRs can be divided into three classes: </P> <Ol> <Li> signaling PRRs that activate gene transcriptional mechanisms that lead to cellular activation, </Li> <Li> endocytic PRRs that function in pathogen binding and phagocytosis, and </Li> <Li> secreted PRRs that usually function as opsonins or activators of complement . </Li> </Ol>

Where would i find a dust cell and what would it be doing