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blood is a fluid connective tissue because of its mesenchymal origin and low cell to liquid intercellular substance (blood plasma) in human adult about 5 liter \(7-8%) of the weight. blood made up of two parts: formed elements (blood cells) and plasma plasma: the liquid in which formed elements suspended  pathology of WBCs: leukopenia: is a decrease in white blood cell number. its a characteristics of the initial stage of viral infection. it may also be due to other condition. leukocytosis: is an increase in the number of leukocytes, principally neutrophils, and it may occurs as a response to bacterial infection. leukemia: is a pathologic increase in number of WBCs. the term denotes a type of neoplastic disease.