All immune cells come from precursors in the bone marrow and develop into mature cells through a series of changes that can occur in different parts of the body.

Skin: The skin is usually the first line of defense against microbes. Skin cells produce and secrete important antimicrobial proteins, and immune cells can be found in specific layers of skin.

Bone marrow: The bone marrow contains stems cells that can develop into a variety of cell types. The common myeloid progenitor stem cell in the bone marrow is the precursor to innate immune cells—neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages—that are important first-line responders to infection.

The common lymphoid progenitor stem cell leads to adaptive immune cells—B cells and T cells—that are responsible for mounting responses to specific microbes based on previous encounters (immunological memory). Natural killer (NK) cells also are derived from the common lymphoid progenitor and share features of both innate and adaptive immune cells, as they provide immediate defenses like innate cells but also may be retained as memory cells like adaptive cells. B, T, and NK cells also are called lymphocytes.

Bloodstream: Immune cells constantly circulate throughout the bloodstream, patrolling for problems. When blood tests are used to monitor white blood cells, another term for immune cells, a snapshot of the immune system is taken. If a cell type is either scarce or overabundant in the bloodstream, this may reflect a problem.

Thymus: T cells mature in the thymus, a small organ located in the upper chest.

Lymphatic system: The lymphatic system is a network of vessels and tissues composed of lymph, an extracellular fluid, and lymphoid organs, such as lymph nodes. The lymphatic system is a conduit for travel and communication between tissues and the bloodstream. Immune cells are carried through the lymphatic system and converge in lymph nodes, which are found throughout the body.

Lymph nodes are a communication hub where immune cells sample information brought in from the body. For instance, if adaptive immune cells in the lymph node recognize pieces of a microbe brought in from a distant area, they will activate, replicate, and leave the lymph node to circulate and address the pathogen. Thus, doctors may check patients for swollen lymph nodes, which may indicate an active immune response.

Spleen: The spleen is an organ located behind the stomach. While it is not directly connected to the lymphatic system, it is important for processing information from the bloodstream. Immune cells are enriched in specific areas of the spleen, and upon recognizing blood-borne pathogens, they will activate and respond accordingly.

Mucosal tissue: Mucosal surfaces are prime entry points for pathogens, and specialized immune hubs are strategically located in mucosal tissues like the respiratory tract and gut. For instance, Peyer’s patches are important areas in the small intestine where immune cells can access samples from the gastrointestinal tract.