Red blood cells carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed. Having too few red blood cells in the body (anemia) can make you feel tired, weak, and short of breath because your body tissues are not getting enough oxygen.
Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in stopping bleeding by plugging up holes in blood vessels. Having too few platelets (thrombocytopenia) may cause you to bleed or bruise easily.
White blood cells help the body fight infections. Having too few white blood cells weakens your immune system and can make you more likely to get an infection.