Cytotoxic t-cells (cytotoxic t lymphocytes) cytotoxic t-cells (cytotoxic t lymphocytes) are the only leukocytes that have an important action inside the bloodstream represent 14% of all wbcs attack antigens directly differentiate and produce antibodies that bind to antigens are robust phagocytic cells, but what they phagocytize is unknown

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.