The Organization of Immune Function

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Source: OpenStax Anatomy and Physiology

OpenStax Anatomy and Physiology

The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:

• Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues.

• The rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors.

• The slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses.

The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells. In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function:

• Phagocytic cells, which ingest pathogens to destroy them.

• Lymphocytes, which specifically coordinate the activities of adaptive immunity.

• Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses.


Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., … DeSaix, P. (n.d.). Anatomy and Physiology. Houston, Texas: OpenStax. Access for free at: