Chapter 2 - Components of the Immune System

Abstract

In this chapter, we describe the immune system as a partnership of the circulatory system, lymphatic system, lymphoid organs and tissues, and hematopoietic cells moving among them. Hematopoietic cells in the bone marrow generate myeloid, lymphoid and mast cell progenitors that differentiate into mature leukocytes. Homeostasis is achieved by a balance of hematopoiesis and apoptosis. The primary lymphoid tissues are the bone marrow and thymus. The secondary lymphoid tissues are the lymph nodes, spleen, and mucosa-associated and skin-associated lymphoid tissues (MALT, SALT). Lymphocytes develop in primary lymphoid tissues and are activated by antigen in secondary lymphoid tissues. Tertiary lymphoid tissues are inflamed and infiltrated by effector lymphocytes. Receptor-mediated intracellular signaling is initiated by antigen, PAMPs, DAMPs, cytokines or other ligands and triggers cell differentiation. Cytokines drive signaling that induces innate and adaptive responses. Leukocyte extravasation is mediated by complementary adhesion molecules upregulated by inflammation. Lymphocyte recirculation allows naïve lymphocytes to patrol common sites of antigen entry.