And what that means is antibodies will combine with their complementary antigens and they will basically clump together and the blood will be rejected during that transfusion process. So this is because people carry antibodies for antigens they don't have. And so mixing red blood cells that have particular antigens with complementary antibodies will cause them to stick together causing the process of Agglutination. So to see exactly what we mean, let's take a look at the following table which basically describes which blood types we can mix and which blood types we cannot mix. So on this table, this row, the red row basically describes the blood of that individual that we are donating. While this column, the blue column basically describes the blood of that individual that is receiving that donating blood.