Jan. 31, 2007

How Are Antibodies Used for Blood Typing?

What are antibodies? How are they formed? How can they be used to identify different types of cells? This project is a practical introduction to the immune system.

Objective

This project has two objectives:

1. Identify the unknown "blood types" of the synthetic samples.
2. Understand the importance of blood typing when transfusions are necessary for medical treatment.

Introduction

The human immune system has various ways of responding to an infection caused by bacteria or viruses. Our bodies produce proteins (antibodies) that are highly specific for the infectious agent as a part of our "humoral" immune response. The antibodies help stop the infection from spreading further and help to eliminate the bacteria or virus from the body.

Antibodies are also used to help our bodies find and destroy "foreign" cells such as tumors. Because antibodies bind tightly to only one type of structure on the surface of cells (antigen), they can also be useful for identifying different types of blood cells. It is important to correctly identify blood cells in our bodies if we ever need to receive blood from someone else because we are sick (transfusion).

Red blood cells, as seen highly magnified under an electron microscope.

CDC/PHIL

Our blood type is determined based on the presence or absence of two proteins on the surface of our red blood cells (Type A and Type B). There are four possible combinations of blood types, namely Type A, Type B, Type AB, and Type O (contains neither A nor B proteins). This is referred to as the ABO blood type. In addition, red blood cells have a Rhesus factor or Rh, which is either present or absent. If the Rh factor is present, the cells are referred to as Rh positive. Including both the ABO and Rh systems for blood typing, there are a total of 8 possible blood types, which are shown in Table 1. The table also shows approximately what percentage of the U.S. population has each blood type.

Table 1: Blood Types and Proportion of U.S. Population for Each Type

Blood Type	% U.S. Population
O positive	38
A positive	34
B positive	9
O negative	7
A negative	6
AB positive	3
B negative	2
AB negative	1

Blood types are determined by using antibody reagents that specifically react with the A, B, and Rh proteins on the surface of red blood cells. First, three drops of blood are placed on a microscope slide. Next, a drop of anti-A reagent is added to one drop of blood, a drop of anti-B reagent is added to the second drop of blood, and a drop of anti-Rh reagent is added to the third drop of blood. The slide is gently rotated and examined for clumping (agglutination). If clumps are seen in the anti-B and anti-Rh reagents, then the person's blood is considered "B positive."

When you donate blood, your blood type is determined (usually by the American Red Cross) and is used to match your blood with someone who needs it. If someone got the wrong blood type during a transfusion, they could have a very severe reaction. Type O negative blood is considered the "universal donor" because anyone can receive that blood type without having a reaction. Type AB positive is considered the "universal recipient" because someone with that blood type can receive blood from anyone else without having a reaction.

Experimental Procedure and Additional Information

Used with permission. Copyright © 2002-2007 Kenneth Lafferty Hess Family Charitable Foundation. All rights reserved.

Sohn, Emily. 2004. Crime lab. Science News for Kids (Dec. 15). Available at http://www.sciencenewsforkids.org/articles/20041215/Feature1.asp .

For more science project ideas, go to http://www.sciencebuddies.org/mentoring/areas_of_science.shtml .

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