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In the past, synthetic materials like Teflon and Dacron have also been used in bypass operations, but they're only helpful for large vessels like the aorta. For a narrower vessel like the coronary arteries, you'd need something else. Think about products that are compatible in the body, that the body wouldn't reject. Also, extremely importantly, that they can't collapse and that that it must be non-thrombogenic, which requires an "endothelial-like" inner lining. It also must have mechanical strength, i.e. a burst pressure, sufficient to operate at arterial pressures. Ideally, however, it must be more than this. It also must have viscoelastic properties that match those of the native vessel being replaced. Finally, if it is to be able to adapt to changing blood flow conditions, it must exhibit vasoactivity, a function which in and of itself can be viewed as biomechanical in nature. To achieve this requires having, as part of the construct, vascular smooth muscle cells, which are contractile in nature and oriented in a circumferential direction.
Putting all these ideas together, perhaps a polymer of some sort (recall that the immune system can't reject polymers (plastics)), plastics can be flexible (elastic), etc.
Second question, line of defense:
1. The first line of defense is nonspecific and part of our innate immunity. It is
chiefly composed of external barriers to pathogens, especially the skin and
mucous membranes.
2. The second line is also our innate immunity. It is internal and is composed of
protective cells, bloodborne chemicals, and processes that inactivate or kill
invaders.
Leukocytes are
divided into two groups according to their appearance in stained blood smears:
• Granulocytes have large granules that stain different colors. Of these, basophils stain blue with basic dye, eosinophils stain red to orange with the acidic dye eosin, and neutrophils stain lilac with a mixture of basic and acidic dyes. Basophils function to release histamine during inflammation, whereas eosinophils and neutrophils phagocytize pathogens. They exit capillaries by squeezing between the cells in a process called diapedesis or emigration.
• Agranulocytes do not appear to have granules when viewed via light microscopy; however, granules become visible with electron microscopy. They are of two types: lymphocytes are the smallest leukocytes and have nuclei that nearly fill the cell, whereas monocytes are large with slightly lobed nuclei. The latter leave the blood and mature into macrophages, which are the phagocytic cells of the second line of defense. Wandering macrophages perform their scavenger function while traveling throughout the body, whereas fixed macrophages do not wander. For example, alveolar macrophages remain in the lungs, and microglia in the central nervous system.
• A special group of phagocytes, the dendritic cells, are located throughout the body, particularly the skin and mucous membranes. Analysis of blood is a key tool of medical diagnosis. The proportions of leukocytes, as determined in a differential white blood cell count, can serve as a sign of disease.
3. The third line of defense is adaptive immunity.
• Antigen presenting cells (B cells)
• T Cells (Cytotoxic T, and helper T cells)
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