The cellular portion of blood contains red blood cells (RBCs), white blood cells (WBCs) and platelets. The RBCs carry oxygen from the lungs ; the WBCs help to fight infection; and platelets are parts of cells that the body uses for clotting. All blood cells are produced in the bone marrow . As children, most of our bones produce blood. As we age this gradually diminishes to just the bones of the spine (vertebrae), breastbone (sternum), ribs, pelvis and small parts of the upper arm and leg. Bone marrow that actively produces blood cells is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow. The process by which the body produces blood is called hematopoiesis . All blood cells (RBCs, WBCs and platelets) come from the same type of cell, called the pluripotential hematopoietic stem cell . This group of cells has the potential to form any of the different types of blood cells and also to reproduce itself. This cell then forms committed stem cells that will form specific types of blood cells.
One of the most recently proposed drivers of the evolution of skin pigmentation in humans is based on research that shows a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum (SC) and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermis , because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented peoples. 
Skin is made up of three layers. The outermost is the epidermis. This consists mainly of cells called keratinocytes, made from the tough protein keratin (also the material in hair and nails). Keratinocytes form several layers that constantly grow outwards as the exterior cells die and flake off. It takes roughly five weeks for newly created cells to work their way to the surface. This covering of dead skin is known as the stratum corneum, or horny layer, and its thickness varies considerably, being more than ten times thicker on the soles of the feet than around the eyes. The epidermis harbors defensive Langerhans cells, which alert the body's immune system to viruses and other infectious agents.