Vasculature invasion

Blood vessels penetrating tumors provide malignant cells with another point at which to enter the circulation. Evidence exists that in situation where cancers disseminate predominantly by the blood, the extent of metastasis depends upon the vasculature of the primary tumor. Thin-walled capillaries, especially those newly formed, provide poor resistance to invading cancer cells. Also, data from microscopy studies show that the endothelium of tumor vessels, particularly in areas of poor oxygenation, is often abnormal (Kl). These abnormalities may permit invasion by neoplastic cells (P3). Finally, tumors can spread by direct extension into body cavities such as pleural and peritoneal spaces. An example of this is the formation of peritoneal metastases from ovarian carcinoma. 

Injury of any tissue gives rise to an immediate inflammatory response and it is probable that this initial response is primarily aimed at priming the process of repair. It is likely that adaptive immunity has evolved in the context of tissue damage induced by infection and the production of an immune response is seen to be most efficient in those tissues most likely to be at risk of a primary infection, e.g., the nasal-oral mucosa, the respiratory, intestinal, and genitourinary surfaces, and the dermis. The CNS is not a site of primary infection and the invasion of the CNS by organisms following systemic infection is limited, presumably by virtue of the properties of the brain vasculature, and when it occurs it seems to reflect more the adaptive ability of the infective agent. 

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