Fig. 1.3 All the cellular elements of the blood, including the cells of the immune system, arise from pluripotent hematopoietic stem cells in the bone marrow. these pluripotent cells divide to produce two types of stem cells. A common lymphoid progenitor gives rise to the lymphoid lineage (blue background) of white blood cells or leukocytes—the innate lymphoid cells (IlCs) and natural killer (nK) cells and the t and B lymphocytes. A common myeloid progenitor gives rise to the myeloid lineage (pink and yellow backgrounds), which comprises the rest of the leukocytes, the erythrocytes (red blood cells), and the megakaryocytes that produce platelets important in blood clotting. t and B lymphocytes are distinguished from the other leukocytes by having antigen receptors and from each other by their sites of differentiation—the thymus and bone marrow, respectively. After encounter with antigen, B cells differentiate into antibody-secreting plasma cells, while t cells differentiate into effector t cells with a variety of functions. unlike t and B cells, IlCs and nK cells lack antigen specificity. the remaining leukocytes are the monocytes, the dendritic cells, and the neutrophils, eosinophils, and basophils. the last three of these circulate in the blood and are termed granulocytes, because of the cytoplasmic granules whose staining gives these cells a distinctive appearance in blood smears, or polymorphonuclear leukocytes, because of their irregularly shaped nuclei. Immature dendritic cells (yellow background) are phagocytic cells that enter the tissues; they mature after they have encountered a potential pathogen. the majority of dendritic cells are derived from the common myeloid progenitor cells, but some may also arise from the common lymphoid progenitor. monocytes enter tissues, where they differentiate into phagocytic macrophages or dendritic cells. mast cells also enter tissues and complete their maturation there.
Principles of innate immunity.
Viruses Intracellular bacteria Extracellular bacteria, Archaea, Protozoa Fungi Parasites
Log scale of size in meters 10–7 10–6 10–5 10–4 10–3 10–2 (1 cm) Fig. 1.4 Pathogens vary greatly in size and lifestyle. Intracellular pathogens include viruses, such as herpes simplex (first panel), and various bacteria, such as Listeria monocytogenes (second panel). many bacteria, such as Staphylococcus aureus (third panel), or fungi, such as Aspergillus fumigates (fourth panel), tissues, as do some archaea and protozoa (third panel). many parasites, such as the nematode Strongyloides stercoralis (fifth panel), are large multicellular organisms that can move throughout the body in a complex life cycle. Second panel courtesy of dan Portnoy. Fifth panel courtesy of James lok.
can grow in the extracellular spaces and directly invade through 1-2 Anatomic and chemical barriers are the first defense against pathogens.