An individual's blood-type alleles carry instructions that direct construction of a specific set of chemicals that protrude from every red blood cell. These chemicals are called:

Antigens

Explanation:

Antigens can be defined as molecules that can bind to antibodies. Many authors prefer to define it as any substance capable of eliciting a response from the immune system, however there are antigenic substances that react with the antibody but are not able to stimulate its production.

Antigens are usually unknown molecules in the body. Molecules that detach from foreign cells such as bacteria, fungi, protozoa or even viruses can be considered antigens. Toxins released by bacteria are also examples of substances that can bind to antibodies.

When an individual's blood-type alleles have instructions that direct the construction of a specific set of chemicals that protrude from all red blood cells, these chemicals are the antigens.