Because TCRs are not secreted, isolating them and determining their structure was much more difficult, and was accomplished only relatively recently. TCRs turn out to be similar in structure to immunoglobulins, but simpler. The most common type of receptor consists of two chains, called a and b , nearly identical in size. The diversity of antibodies and TCRs posed a fascinating conceptual problem because of the enormous number of different specificities they exhibit and because they must have constant as well as variable regions. We now know that several unusual mechanisms contribute to the generation of this diversity and decrease the number of genes required to attain it.
Some diversity is generated at the protein level.
One important mechanism is the combinatorial joining of H and L chains. Most diversity is generated at the gene level. The answers to the first two questions came with the surprising discovery that more than one gene segment codes for each L and each H chain. The immunoglobulin genes are encoded in three clustered gene families, two for L chains and one for H chains. The k light-chain family in mouse consists of about 250 V L genes and a single C L gene.