Discrepancies in the concentrations of specific blood components, such as leukocytes, defensins, Toll receptors, B cells and T cells, differentiate human blood from animal blood. The closer the animal is to humans in terms of a common ancestor, the more similar blood becomes.
Animals with a distant common ancestor, such as arthropods and mollusks, have circulatory systems that function completely differently from their human counterparts. Consequently, the blood of these simple invertebrates is easy to distinguish from human blood.
All vertebrates have blood that contains three main types of cell: red blood cells, white blood cells and platelets. Mature human red blood cells are different from those of non-mammalian animals in that they lack a nucleus. These red blood cells shed the nucleus and organelles once they differentiate and mature to increase their hemoglobin capacity and eliminate their need for oxygen, enabling circulation that is more efficient.
Platelets and white blood cells do not vary much between humans and animals. The types of white blood cell that are present in human blood may differ slightly from one species to another, with humans having five types of white blood cell, while most fish only have four types. Platelets vary in adhesiveness between species, with human platelets being moderately adhesive.