In the ABO blood group system, the grouping of a person's blood is based mainly on the presence or absence of which substances on the surface of red blood cells?

Introduction / Context:
The ABO blood group system is one of the most important classifications in human blood transfusion. It divides people into groups A, B, AB, and O. Understanding what this grouping is based on helps explain why some blood transfusions are safe and others can be dangerous. This question asks which substances determine a person's ABO blood group by being present or absent on the surface of red blood cells.

Given Data / Assumptions:

• The focus is on the ABO blood group system.
• The options mention antibodies, antigens, antitoxins, and enzymes.
• We assume basic knowledge that blood has red blood cells with surface molecules and plasma containing antibodies.
• The question specifically asks what the grouping is based on.

Concept / Approach:
Blood grouping in the ABO system is determined by antigens on the red blood cell membrane, known as A and B antigens. A person with antigen A is group A, with antigen B is group B, with both antigens is group AB, and with neither is group O. The plasma contains antibodies against the antigens the person does not have. Antitoxins are specific antibodies against toxins, and enzymes are not used to label ABO groups. Therefore, the grouping is based primarily on antigens present on red blood cells, not on antibodies or enzymes.

Step-by-Step Solution:
Step 1: Recall that red blood cells may carry antigen A, antigen B, both, or neither on their surfaces. Step 2: Recognise that these antigens are glycoproteins or glycolipids that act as identification markers for the immune system. Step 3: Understand that blood group A has antigen A, group B has antigen B, group AB has both, and group O has neither. Step 4: Note that antibodies are indeed important, but they are present in the plasma and form as anti A or anti B depending on the person's own antigens. Step 5: Since the question asks what the grouping is based on, choose antigens present on the red blood cell membrane.

Verification / Alternative check:
Standard diagrams of the ABO system in textbooks show red blood cells with labels A, B, AB, and O, where the letters refer to surface antigens. The accompanying text explains that plasma contains antibodies against missing antigens, such as anti B in group A blood. Laboratory blood grouping tests use antibodies to detect the presence of antigens on red cells, again confirming that antigens are the basic markers for grouping. This consistent presentation makes it clear that ABO grouping is based on red cell antigens.

Why Other Options Are Wrong:
Antibodies present in the blood plasma are wrong as the primary basis of grouping because they are formed in response to antigens and are used to cross match blood, but they do not define the group itself. Antitoxins produced against bacterial poisons are specific antibodies used to neutralise toxins and are unrelated to ABO grouping. Enzymes involved in blood clotting, such as those in the coagulation cascade, have nothing to do with ABO blood group classification.

Common Pitfalls:
Students sometimes get confused because practical blood grouping tests involve mixing blood with known antibodies to see whether clumping occurs. This may make it seem as if antibodies are the main basis. In reality, the test uses antibodies to detect which antigens are present. To avoid confusion, remember that group names A, B, AB, and O refer to antigens on red blood cells, and antibodies in plasma are secondary, reacting against antigens that are not self.

Final Answer:
The correct option is Antigens present on the red blood cell membrane, because ABO blood grouping is fundamentally based on which antigens are present or absent on the surface of red blood cells.