Anatomy of the Immune System

Skin

Usually the first line of defense against pathogens; cells produce and secrete important antimicrobial proteins, and immune cells can be found in specific skin layers.

Epithelial Membranes

Epithelial membranes line the respiratory tract, gastrointestinal tract, and genitourinary tract: areas that constitute prime entry points for pathogens. The body defends these surfaces by producing secretions, such as mucus or saliva, which contain antimicrobial substances.

Bone Marrow

Contains stem cells that can develop into a variety of blood cell types, such as red blood cells, platelets, and lymphocytes among others.

Lymph Nodes

Substances transported by lymph pass through lymph nodes. In the lymph nodes, white blood cells can collect, interact with each other and with antigens, and generate immune responses to foreign substances. Lymph nodes contain a mesh of tissue that is tightly packed with B cells, T cells, dendritic cells, and macrophages. Harmful pathogens are filtered through the mesh, then identified and attacked by B cells and T cells.

Other Sites

Other organs and collections of immune tissue include the spleen, tonsils, Peyer's patches, and the small intestine.