Immunology Course Outline

Immunology Course Outline - Session 1

The Cast of Characters - Review of Histology, Anatomy, and Terminology

Cells of the Immune System

The predominant cells of the immune system are the white blood cells (WBCs). The most numerous of these are the granulocytes and the lymphocytes. These cells have molecules on their surfaces that serve a variety of functions. Some of these molecules are used as antigenic markers to identify the cell type. The most common markers are called "clusters of differentiation" markers, or more commonly CD markers. There are now hundreds of these CD markers known.

Often, the cells are identified by morphology alone, as seen on a peripheral blood smear or bone marrow biopsy and smear. However, CD markers are needed to distinguish the various lymphocyte populations, since the cells may look similarly based on morphology alone.

Granulocytes
	Neutrophils - Are the most numerous granulocyte in circulation. They are short-lived, lasting for no more than a day, usually only hours. They react to a variety of non-specific stimuli. They tend to be involved in acute inflammatory reactions. Granulocytes are often the first cell to arrive on the scene. Their granules are lysosomes that contain a variety of substances that mediate immune responses. The lysosomes can fuse with phagocytized microbes and destroy them. Animation: process of phagocytosis of an infectious agent by a neutrophil
	Eosinophils - Are distinguished by the presence of bright red granules with the Wright-Giemsa stain used on hematologic preparations, giving them their name. They are most often associated with allergic and parasitic immune responses.
	Basophils - Are the least numerous and least understood peripheral blood granulocyte. Like mast cells in the tissues, they have prominent blue to purple cytoplasmic granules that contain a variety of substances, including vasoactive amines. They may function similar to mast cells.

Lymphocytes
	T cells - They have CD3 on their surface; they have T cell receptors (TCR) that recognize specific antigenic sequences presented to them.
		CD8 cells - restricted to MHC class 1 antigenic interaction; known as "suppressor cells" that have a cytotoxic effect.
		CD4 cells - restricted to MHC class 2 antigenic interaction; known as "helper cells" because they assist in antigen processing and stimulation of immune responses.
	B cells - Most have CD19 and CD20 on their surface. They have surface immunoglobulin that is specific for particular antigenic sequences. Fully differentiated immunoglobulin producing B cells are known as plasma cells.
	NK cells - These "natural killer cells" are part of the innate immune response and react to stimuli that are not antigen specific. They can express CD11 and CD12 on their surface.

Monocytes - Macrophages - Monocytes are formed in the bone marrow and circulate. They can leave the bloodstream when attracted by inflammatory responses and become tissue macrophages. Macrophages can live for months. Macrophages have a phagocytic function to clean up the mess left after an inflammatory response. Macrophages are a type of antigen presenting cell, serving to process the phagocytized material and then present peptides to lymphocytes.

Animation: process of phagocytosis of an infectious agent by a macrophage

Mast Cells - They are not numerous, but are strategically positioned around blood vessels in connective tissues of the body, particularly in the submucosa of the respiratory and gastrointestinal tracts and in the dermis. They are large cells with many prominent granules that are filled with many active materials, including vasoactive amines such as histamine. They participate in immediate hypersensitivity reactions (a form of allergic reaction).

Dendritic Cells - These are obscure cells that are hard to see in regular histologic tissue sections. They have many long cytoplasmic processes that aid in capturing antigenic materials and then presenting them to lymphocytes. Thus, they are a form of antigen processing cell. Those located in epithelia are known as Langerhans cells; those in lymphoid follicles are known as follicular dendritic cells.

Stem cells - They are found in the bone marrow and give rise to lymphoid, myeloid, and erythrocytic cell lines. A few of them circulate. They are CD34 positive.

CD Marker	Cells Marked
CD1a	Antigen presenting cells: Langerhans cells, dendritic cells
CD3	Mature T lymphocytes and NK cells
CD4	Helper T lymphocytes
CD8	Suppressor T lymphocytes
CD10	CALLA antigen of lymphoid progenitors
CD11	Many leukocytes
CD13	Myeloid cells
CD15	Reed-Sternberg cells of Hodgkin disease; some myeloid cells
CD19	B lymphocytes
CD20	B lymphocytes
CD22	B lymphocytes
CD33	Myeloid progenitors
CD34	Hematopoiestic progenitors (stem cells), endothelial cells
CD45	All leukocytes (common leukocyte antigen)
CD45RO	T cells
CD56	NK cells
CD64	Mature myeloid cells
CD132	Common gamma chain
CD154	CD40 ligand