The answer is a process that selects variation that help survival and that spreads the variation to more offsprings.
The idea is that there is a higher chance of survival if the organisms has certain characteristics and it is more likely to pass on their genes of this characteristic to the next generations.
For example, there's dark colored fish and white fish. In the deep ocean, dark colored fish can camouflage themselves. However, white fish can be seen easily by predators. Therefore, eventually more dark colored fish survives and more offsprings are dark ones.
As food moves through the digestive tract, it spends the most time in the large intestines. This is because of the need to absorb water and partially digest fiber.
Helper T cells are the CD4T cells that are present in the inactive form. These cells have receptors for antigens on their surface. These antigen receptors are called T-cell receptors (TCRs). The function of TCR is to recognize and bind to specific foreign antigen fragments that are presented in antigen–MHC complexes. Different helper T cells have their unique TCRs to recognize a specific antigen–MHC complex. The CD4 proteins of helper T cells also interact with the MHC antigens and help maintain the TCR–MHC coupling.
