Final answer:
The second checkpoint in T cell development tests whether the newly formed TCR can bind to self-antigens. This is a crucial part of ensuring self-tolerance and preventing autoimmunity. Helper T cells bind to antigens presented with MHC II molecules.
Step-by-step explanation:
The second checkpoint in T cell development tests whether the newly formed TCR can bind to self-antigens. During T cell development, T cell receptors (TCRs) are formed and must be tested for their ability to recognize self-antigens presented by MHC molecules, which is essential for ensuring self-tolerance and preventing autoimmunity. This process is known as positive and negative selection, which takes place in the thymus. Mature T cells then express either CD4 or CD8 and their TCRs can bind to antigens presented with MHC I or MHC II molecules, depending on the cell type.
Helper T cells, for example, have TCRs that bind to antigens presented with MHC II molecules. These TCRs are highly specific and enable an immune response through the process of clonal selection and expansion. Mechanisms of peripheral tolerance also function to further modulate immune responses after T cells have matured and exited the thymus. Understanding the specificity of TCRs and their interaction with MHC is crucial in comprehending how adaptive immunity operates and how T cells assist in immune responses by recognizing and responding to foreign antigens.