Overview of the immune and hematopoietic systems.

Current knowledge of the immune and hematopoietic systems is reviewed. All blood cells are derived from the totipotent stem cell, also known as the pluripotent stem cell. The differentiation of pluripotent peripheral stem cells into blood cells is controlled by a variety of biologic response modifiers, including colony-stimulating factors (CSFs) and interleukins. Among the known CSFs are stem cell growth factor, granulocyte-macrophage CSF, multilineage CSF (interleukin-3), granulocyte CSF, macrophage CSF, and erythropoietin. CSFs are categorized as class I (those that stimulate the production of several types of blood cells; also called pluripotent) and class II (those that stimulate only one cell line; also called unipotent). Effects of CSFs can be studied using laboratory tests of colony-forming-unit activity. Pathogens entering the body through damaged skin or mucous membranes are met with both a cellular response (neutrophils, macrophages, cytotoxic T lymphocytes, and natural killer cells) and a humoral response (antibodies and complement). There is interplay between these two arms of the immune system to defend against foreign antigens. This interplay can occur by cell-to-cell contact and by cytokines. Hematopoietic and immune cells of the body are produced and destroyed under precise control of many different biologic response modifiers, including the colony-stimulating factors, interleukins, and interferons.