Donating blood to the needy is a noble act of saving lives. Most of us are ready to do so. But the doctor first ensures the type of blood we carry, since our blood comes in various physiological types or groups. The general types are A, B, AB and 0. Matching of the blood types of the donor and the recipient is vital, else it could lead to serious life-threatening complications. Most of are not aware of a rare blood group hh, which was discovered in Bombay in 1952, and hence known as Bombay Blood. People who carry this rare blood type, about 1 in 10, 000 Indians, can accept blood only from another Bombay Blood type individual, and not from anyone who is 0, A, B or AB type. To understand the biology behind this exceptional blood type, let us first look at what each blood group type contains. Blood contains red blood cells (and other cells that are not relevant for us here, floating in a fluid called plasma. Red blood cells carry on their surface a set of markers with which the plasma interacts. This compatibility or cross-talk between the cell and the plasma is what makes each blood type special. The markei a on the cell are determined by a master type called H, out of which are generated types A, B, AB and AO. When blood transfusion occurs, the compatibility between the donor blood cell type and the plasma of the recipient becomes vital, else the blood may coagulate or clump up, causing serious danger. Thus people with A type blood can accept blood from A type or 0 type and not from AB or B type donors. And they can donate, in turn, to those with A or AB types. Those with B type can receive from B or 0 types, and can donate to those with B or AB types. Those with AB type blood can receive from any one and donate to other AB. Finally those with 0 type blood can receive only from 0, but can donate their blood to A, B, 0 or AB- universal donors. In contrast, the Bombay doctors found that the hh type (Bombay type people) can accept only from other hh type. This makes the Bombay Blood types a very special and rare category of people. People are so rare with this blood group because of extensive inbreeding within the same lineage or close-community marriages, often consanguineous, such that the blood type or the gene pool is greatly restricted. Such intra-community marriages have happened in small isolated communities such as the gypsies, Russian Jewish or Parsi communities. It is thus likely that the Bombay Blood types have common ancestral origins.