Maximization of surface area is important in many biological situations. Give examples in digestion, circulation and respiration. O2 binding to hemoglobin is saturable. Compare the daily volume of fluid the kidneys filter with the daily volume of urine the kidneys produce. Urine is sterile in a human with healthy kidneys.
Cells involved in the immune response are white blood cells.
Following activation by a specific antigen and subsequent clonal selection, B-cell progeny produce free IgG antibodies with nearly the same structure as their receptors. There are more than 1,000,000 different B-cell receptors, recognizing an equivalent number of different potential antigens. However, there are only about 100,000 different genes in us.