What is clonal selection?

Clonal selection is a part of human immune response where specific B or T-helper lymphocytes are chosen to undergo clonal expansion

B-lymphocytes and T-helper lymphocytes have unique and specific antigen receptors on their cell surface membranes. Each cell will have a slightly different type of antigen receptor. These cells will interact with pathogen antigen presenting cells, such as the pathogen iteslf, macropahges (that have ingested a pathogen and present the pathogens antigens on it's own cell surface membrane), or infected human cells, to see if their antigen receptors match the antigens. Once a match has been found, the cell that has the correct antigen receptor will undergo clonal expansion, where the cell will makes millions of copies of itself through mitosis. 

The principle is important because B-lymphocytes eventaully differentiate and go on to create unique antibodies that target the specific antigens on pathogens and destroy them to fight the infection. Large amounts of energy and resources would be wasted creating antibodies that don't fit antigens on the pathogen if T or B lymphocytes without the corresponding antigen receptor were chosen to replicate/go through clonal expansion.