FIGURE 14.1  Overview of the structures constituting the lymphatic system. Because lymphatic tissue is the main component of some organs, they are regarded as organs of the lymphatic system (spleen, thymus, lymph nodes). Lymphatic tissue is present as part of other organs such as red bone marrow, lymphatic nodules of the alimentary canal: tonsils, vermiform appendix, gut-associated lymphatic tissue (GALT) and of the respiratory system (bronchus-associated lymphatic tissue, or BALT ), and, not shown in the illustration, diffuse lymphatic tissue of mucous membranes (mucosa-associated lymphatic tissue, or MALT). The lymph nodes are interspersed along the superficial lymphatic vessels (associated with the skin and superficial fascia) and deep lymphatic vessels (associated with main arteries). Ultimately, the lymphatic vessels empty into the bloodstream by joining the large veins at the base of the neck. The thoracic duct is the largest lymphatic vessel.
The immune responses can be divided into nonspecific (innate) and specific (adaptive) defenses.
The body has two lines of immune defenses against foreign invaders and transformed cells: nonspecific immunity and specific immunity.
 In nonspecific (innate) immunity, preexisting nonspecific defenses constitute the innate immune response. In all living organisms, the innate immunity represents the frst line of defense against microbial aggression. It consists of (1) physical barriers (e.g., the skin and mucous membranes) that prevent foreign organisms from invading the tissues, (2) chemical defenses (e.g., low pH ) that destroy many invading microorganisms, (3) various secretory substances (e.g., thiocyanate in saliva, lysozymes,  interferons, fbronectin, and complement in serum) that neutralize foreign cells, and (4) phagocytic cells (e.g., macrophages, neutrophils, and monocytes) and natural killer (NK) cells.
 With specific (adaptive) immunity, if nonspecific defenses fail, the immune system provides specific, or adaptive, defenses that target specific invaders. The initial contact with a specific antigen or foreign agent initiates a chain of reactions that involve effector cells of the immune system and frequently leads to a state of immune “memory.” Adaptive immunity induces acquired resistance against microbial aggression through random somatic rearrangements of genes that encode immunoglobulins and specifc receptors on T lymphocytes (i.e., T-cell receptors, or TCRs). During adaptive immune responses, specifc B and T lymphocytes become activated to destroy invading organisms. Two types of specific defenses have been identified: Humoral response results in the production of proteins called antibodies that mark invaders for destruction by other immune cells, and the cellular immune response targets transformed and virus-infected cells for destruction by specific “killer” cells.
Therefore, shortly after invasion by bacteria or other pathogenic agents, the immune system becomes activated (inflammatory response) to destroy infectious agents and generate long-term memory against pathogens.
Cells of the immune system include lymphocytes and various supporting cells.