Natural antibodies (NAb) are defined as those immunoglobulins (mainly IgM isotype) continuously secreted by B1 type lymphocytes without any previous external antigen stimulation. To date, a broad range of NAb target carbohydrates (NAbC) have been characterized and can be divided into three different groups: 1) conservative NAbC that are practically the same regarding specificity and blood levels in all healthy individuals; 2) alloantibodies to blood group antigens and xenoantibodies to antigens from other species; 3) plastic antibodies, which include antibodies from the two previous groups but with variable levels during certain conditions, such as pregnancy or inflammation.