Connective tissue includes liquid blood, fibrous intrinsic connective tissue, and solid cartilage and bone tissue. The sparse connective tissue is widely distributed and exists between organs and various tissues with loosely arranged cells and intercellular matrix, and its main functions are connectivity, nutrition, protection and defense. There are many types of cells in lax connective tissue, mainly fibroblasts, adipocytes, macrophages, plasma cells and mast cells. Fibroblasts are capable of synthesizing proteins; adipocytes are large and round, with a large number of fat droplets stored in the cytoplasm, which can store fat; macrophages are capable of engulfing foreign bodies and senescent cells and are involved in the immune response; plasma cells are capable of synthesizing immunoglobulins; and mast cells are involved in the allergic response in the body.