Final answer:
The two major families of TCR are alpha-beta and gamma-delta, with alpha-beta being the most common and important for adaptive immunity, recognizing antigens presented by MHC molecules. Alpha-beta TCRs are found on the majority of T cells, while gamma-delta TCRs represent a smaller fraction and are associated with innate-like immune responses.
Step-by-step explanation:
The two major families of TCR are the alpha-beta (αβ) and gamma-delta (γδ) T-cell receptors. The αβ T-cell receptors are the most common and pivotal in the adaptive immune response. These receptors consist of two peptide chains, known as the alpha (α) and beta (β) chains, both of which span the cytoplasmic membrane of the T cell. They recognize processed antigens that are presented by Major Histocompatibility Complex (MHC) class I or MHC class II molecules.
In comparison to B-cell receptors and antibodies like IgD and IgM, TCRs are smaller and simpler, with a variable and a constant region but lacking the four-chain, Y-shaped structure of immunoglobulins. Instead, TCRs have a unique two-chain structure enabling their function. The variable regions of these chains provide the antigen-binding site. The diversity of these regions is achieved through a process of genetic rearrangement in which variable (V), diversity (D), and joining (J) gene segments are shuffled and combined during T cell development in the thymus.
While exact ratios can vary, the majority of T cells express the αβ TCR. The gamma-delta (γδ) TCRs are less common and associated with innate-like responses and are found in specific tissue sites. The αβ T cells can be further characterized by CD4 and CD8 markers, indicative of helper T cells and cytotoxic T cells, respectively.