not specific for any infectious agent and do not appear to improve response upon repeated exposure

is due to the genetic and constitutional characteristics of a person

preliminary contact with microorganisms or their metabolic products is not necessary

develops to protect the host from the harmful effects of foreign substances

acts predominantly against extracellular pathogens and intracellular pathogens

an encounter with an agent usually causes a complex reaction

differentiation of the common lymphoid progenitor

major organ of lymphopoiesis at embryonic stage and to some degree at the adult stage

a group of lymphatic follicles found in the lamina propria of the small intestine (ileum) extending into its submucosa

located symmetrically at various places in the body, are "customs barriers" for all "regional" antigens

their lymphocytes surround arterioles forming a cuff or so-called periarteriolar lymphoid sheaths

contains lobules separated by trabeculae