Organization of Immune Function

The Immune System

• The immune system is a complex collection of barriers, cells, and soluble proteins that interact and communicate with each other.
• The modern model of immune function is organized into three phases based on the timing of their effects:
• Barrier defenses (instantaneous)
• Innate immune response (rapid but nonspecific)
• Adaptive immune response (slower but more specific and effective)

Development of Blood Cells

• All blood cells, including those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic 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.
• There are three classes of blood cells based on function:
• Phagocytic cells (ingest pathogens to destroy them)
• Lymphocytes (coordinate adaptive immunity)
• Cells containing cytoplasmic granules (mediate immune responses against parasites and intracellular pathogens)

Lymphocytes

• Lymphocytes are the primary cells of adaptive immune responses.
• There are two basic types of lymphocytes: B cells and T cells.
• B cells mature in red bone marrow, while T cells mature in the thymus gland.
• B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs (spleen and lymph nodes).

B Cells

• B cells function primarily by producing antibodies.
• An antibody is a protein that binds specifically to pathogen-associated molecules (antigens).
• Activated B cells differentiate into plasma cells, which secrete a soluble form of their surface antibodies.

T Cells

• T cells do not secrete antibody but perform a variety of functions in the adaptive immune response.
• Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens.

Plasma Cells

• Plasma cells are activated B cells that have differentiated in response to antigen binding.
• They secrete soluble antibodies and contain a large amount of cytoplasm packed with protein-synthesizing machinery (rough endoplasmic reticulum).

Natural Killer Cells

• Natural killer (NK) cells are participants in the innate immune response.
• NK cells contain cytotoxic (cell-killing) granules in their extensive cytoplasm.
• They share this mechanism with cytotoxic T cells of the adaptive immune response.
• NK cells are among the body’s first lines of defense against viruses and certain types of cancer.

Lymphocyte Types and Functions

• B lymphocyte: generates diverse antibodies
• T lymphocyte: secretes chemical messengers
• Plasma cell: secretes antibodies
• NK cell: destroys virally infected cells

Development and Maturation of B and T Cells

• Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response.
• The primary lymphoid organs are the bone marrow and thymus gland.
• The lymphoid organs are where lymphocytes mature, proliferate, and are selected, enabling them to attack pathogens without harming the body’s own cells.

Thymus Gland

• The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart.
• It is where T cells mature and are selected to attack pathogens without harming the body’s own cells.