1. Kuby Immunology, 7e: Chapter 2
Cells, Organs, and Microenvironments of the Immune System

2. Cells of the immune system
Immune responses result from coordinated activities of many cells, organs, and microenvironments in the body
Hematopoietic stem cells (HSCs) have the ability to differentiate into many types of blood cells
All red and white blood cells develop from a pluripotent HSC during hematopoiesis
Hematopoiesis is a highly regulated process

4. Cells of the immune system
In adult vertebrates, hematopoiesis occurs in the bone marrow
Within the bone marrow, HSCs are constantly renewed and directed to differentiate into two major types of progenitors
Common myeloid progenitor cells
Common lymphoid progenitor cells

5. Cells of the immune system
Four main types of cells develop from common myeloid progenitors
Red blood cells (erythrocytes)
Monocytes
Granulocytes
Megakaryocytes (form platelets―cell fragments placed into circulation for clotting)

6. Cells of the immune system
Four main types of cells develop from common myeloid progenitors
Red blood cells (erythrocytes)
Monocytes
Granulocytes
Neutrophils―direct harm to pathogens
Basophils/mast cells―inflammation/allergies
Eosinophils―antiviral activity, antiparasite activity
Megakaryocytes

9. Cells of the immune system
Four main types of cells develop from common myeloid progenitors
Red blood cells (erythrocytes)
Monocytes
Migrate into tissues and differentiate into macrophages―function to repair/remodel, destroy pathogens, present antigens
Can also differentiate into dendritic cells―HIGH degree of function as ingesters of antigens, followed by presentation to naïve T lymphocytes for initial activation
Granulocytes
Megakaryocytes

11. Cells of the immune system
Macrophages and neutrophils are specialized for phagocytosis
Macrophages can also present antigens to T cells via MHC molecules
Immature dendritic cells capture antigen, then mature and migrate out of that location to another to present antigen to T cells
Dendritic cells are the most potent antigen-presenting cells for activating naïve T cells

12. Cells of the immune system
Three main types of cells develop from the common lymphoid progenitor
B lymphocytes
T lymphocytes
There is a subset that appears more similar to Natural Killer cells that are known as NKT cells
NK cells
Lymphocytes appear very similar, but different sets carry different cluster of differentiation (CD) molecules on their surface

15. Primary lymphoid organs―Where immune cells develop
B lymphocytes develop in the niche of the bone marrow

16. Primary lymphoid organs―Where immune cells develop
T cells develop initially in the bone marrow, but then migrate to the thymus to achieve full maturity

17. Secondary lymphoid organs―Where the immune response is initiated
Areas where lymphocytes encounter antigen, become activated, undergo clonal expansion, and differentiate into effector cells
Secondary lymphoid organ areas include:
Lymph nodes
Spleen
Mucosa-associated lymphoid tissue (MALT)
Other diffuse and loosely organized areas
These are connected to each other via the blood and lymphatic circulatory systems

19. Secondary lymphoid organs―Where the immune response is initiated
Lymph nodes and spleen are the most highly organized secondary lymphoid organs
T-cell and B-cell activity are separated into distinct microenvironments
The cells will actively migrate toward each other during activation events for their required interactions

20. Secondary lymphoid organs―Where the immune response is initiated
Differentiation into effector cells takes place in follicles of secondary lymphoid organs
T lymphocytes
CD4+ T cells differentiate into helper T cells that assist in B-cell differentiation
CD8+ T cells differentiate into killer (or cytotoxic) T cells that attack and destroy virally infected cells

21. Secondary lymphoid organs―Where the immune response is initiated
Differentiation into effector cells takes place in follicles of secondary lymphoid organs
B cells further mature in germinal centers in such tissues
Antigen affinity is increased
Class switching can take place
Both B and T lymphocytes will develop into long-lived memory cells in these areas, as well

24. Secondary lymphoid organs―Where the immune response is initiated
The spleen is the first line of defense against blood-borne pathogens
Red blood cells are compartmentalized in red pulp
White blood cells are segregated in white pulp
A specialized region of macrophages and B cells known as the marginal zone borders the white pulp

26. Secondary lymphoid organs―Where the immune response is initiated
Mucosa-associated lymphoid tissue (MALT)
Important layer of defense against infection at mucosal and epithelial layers
Organizes responses to antigens that enter mucosal tissues
Includes a network of follicles and lymphoid microenvironments associated with the intestines (gut-associated lymphoid tissue, or GALT)

28. Secondary lymphoid organs―Where the immune response is initiated
MALT
M cells in the lining of the gut are unique
They function to deliver antigen from the intestinal spaces to lymphoid cells in the gut wall

29. Secondary lymphoid organs―Where the immune response is initiated
Various loosely organized and diffuse lymphoid tissue are also found under the skin, mucosae, and tertiary tissues at sites of infection

30. Summary
While blood cell development is a necessary part of immune responses, it’s only a first step
Multiple other organs and tissues of the body must receive those blood cells and interface them with each other to achieve proper immune responses
These interfaces and the tissues involved are complex and multifaceted, as shown in this chapter’s materials