## **Core Concept**
ABO blood group antigens are carbohydrate molecules present on the surface of red blood cells and other tissues in the body. These antigens determine an individual's blood type according to the presence or absence of specific antigens. The ABO blood group system includes A and B antigens, and individuals can have A, B, both (AB), or neither (O) antigen on their red blood cells.

## **Why the Correct Answer is Right**
The correct answer, **saliva**, is not entirely accurate as written because some individuals, known as secretors, do have ABO antigens present in their saliva. However, not everyone has ABO antigens in their bodily secretions. The presence of ABO antigens in saliva and other bodily secretions depends on the individual being a secretor, which is determined by a specific gene. Approximately 80% of people are secretors, and they have ABO antigens in their saliva, while about 20% are non-secretors and do not have these antigens in their saliva.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because ABO antigens are indeed found on red blood cells, which is the basis for determining an individual's blood type.
- **Option B:** This option is incorrect as ABO antigens can be found in plasma, where they can form part of the ABO blood group system, influencing compatibility in transfusions.
- **Option C:** This option might seem plausible but given that **D. Saliva** can contain ABO antigens in secretors, it actually refers to a context (saliva) where ABO antigens are variably present.
- **Option D:** Not applicable as D is the correct answer.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that about 80% of the population are secretors and express ABO blood group antigens in their saliva, tears, and other bodily secretions, while 20% are non-secretors and do not express these antigens in their bodily secretions.

## **Correct Answer: D. Saliva**