Integumentary system: skin and its accessory structures such as hair, sebaceous glands, sweat glands and nails. Antigen presenting cells (APCs)- a heterogeneous group of immune cells that mediate the cellular immune response by processing and presenting antigens for recognition by certain lymphocytes such as T cells. Classical APCs include dendritic cells, macrophages, Langerhans cells (macrophages of the skin that can migrate to the lymph vessels) and B cells. Hemidesmosomes- multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. Type I collagen- fibrils form fibers which can form bundles; resists tension (most abundant; in tendons, bones, dermis) Type VII collagen- collagen type essential in strengthening and stabilizing the skin.