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Red blood cells(Erythrocytes) : carrying haemoglobin to the cells of the body

White blood cells(Leucocytes) : Protecting the body against foreign substances(germs). 
1.They engulf germs or
2. synthesize antibodies to kill the germs

Platelets(Thrombocytes) : Help in clotting of blood and prevent excess leakage of blood and prevent haemorrhage. Note: Platelets are not cells. these are fragments of cells