Microtubules are hollow cylinders made up of polymers of the protein tubulin. Microtubules are major components of cilia and flagella, which are tail like projections that are covered by a plasma membrane and extend outwards from the cell. Motile cilia are used for locomotion and food gathering by some protozoa and are found in the lining of the trachea, where their wave like motion propels mucus, dust and other foreign matter out of the lungs.
In most mammals, antibody production does not occur until after birth. The newborn calf receives antibodies in milk and these antibodies are responsible for passive immunity during early postnatal life. Thus, fetal calf serum is devoid of antibodies, neonatal calf serum contains antibodies of maternal origin while serum from the adult contains large amounts of antibodies made by the mature immune system. This fascinating developmental scheme is illustrated in this exercise.