Description and Classification
White blood cells are described and classified in several ways:
1. By function:Defense cells; phagocytes (granulocytes, macrophages) Cells that produce antibodies and cellular immunity (lymphocytes, plasma cells)
2. By the shape of the nucleus: Polymorphonuclear or mononuclear
3. By the site of origin: Myeloid (MYE- = bone marrow), lymphoid (lymph refers to the lymphatic system and the fluid collected from tissues that flows through the lymph vessels and is added to venous blood)
4. By staining characteristics: Granulocytes, non-granulocytes; neutrophils, eosinophils (eosin is an acid dye), basophils
Granulocytes begin with the stem cells. Lymphocytes have their origin with common progenitor cells, the origin
of the stem cells
Granulopoiesis is the formation of granulocytes, the most numerous white cell. As a granulocyte matures, the cell nucleus undergoes many changes; it shrinks, indents, assumes a band form, and segments. Granules containing enzymes and antibacterial agents appear; they are clearly evident in image below. Myelocytes are distinguished according to the staining characteristics of their granules: neutrophilic, eosinophilic, and basophilic. Mature granulocytes are polymorphonuclear cells (PMNs) (sometimes called polys).
Normally, granulocytes are regulated at a constant level. During infection, the number of granulocytes rises dramatically.
Functions of Granulocytes
Neutrophils seek out and kill bacteria – a process called phagocytosis (see table below). Eosinophils attack some parasites and inactivate mediators released during allergic reactions. Basophils contain histamine and are important in immunity and hypersensitivity reactions; they also contain heparin (an anti-clotting substance), but their role in blood clotting is uncertain.
Phagocytes bind to and ingest antigens. (Neutrophils can ingest 5-25 bacteria before dying; macrophages live longer because they can extrude toxic substances.)
After a short time in the blood, monocytes (Figure below) enter tissue, grow larger, and become tissue macrophages. Once it was incorrectly thought that endothelial cells (cells lining blood vessels) performed the same function as macrophages. This is the origin of the name reticuloendothelial system (RES); macrophage system is more accurate.
Activated by the immune system, macrophages are the body’s first line of defense. Powerful and long-lived, macrophages can ingest large particles – whole RBCs, parasites, bacteria – and rid the body of dead or damaged cells and other debris. Macrophages fixed in tissue often look different and have different names, e.g., Kupffer cells in the liver are macrophages.
Macrophages and Neutrophils
Areas of inflammation attract both macrophages and neutrophils. The few tissue macrophages present begin to attack invaders. Within a few hours, the second line of defense appears: the number of neutrophils in the blood increases substantially as substances in the blood stimulate bone marrow to release stored neutrophils. The third line and the long-term chronic defense is the proliferation of macrophages by cell reproduction in the tissue, by attracting monocytes from the blood, and by increased production of monocytes.
Lymphocytes Play a Key Role in Immunity
T lymphocytes (T cells): Involved in cellular immunity, T cells carry receptors for molecules on other cellsor in body fluids. T cell receptors allow them to interact with macrophages and other cells and substances in the body. T cells defend against foreign substances such as viruses that invade body cells, fungi, parasites, transplanted tissue, and cancer cells. Through a variety of T cells (helper T cells, suppressor T cells), the body initiates, carries through, and terminates antigen-antibody reactions to provide immunity.
B lymphocytes (B cells) are involved in humoral immunity, which consists of antibodies circulating in the blood and lymphatic system. B lymphocytes produce antibodies typically against bacteria and viruses. Example: AIDS. In AIDS (acquired immune deficiency syndrome), the virus (HIV) infects the helper T lymphocyte cells and destroys them. It can also pass unrecognized from cell to cell by changing the surface of helper T cells. HIV may lie dormant until another infection triggers T cells to increase and the virus to multiply. With HIV, not only is part of the body’s defense system lost, but the very cells that should defend the body are working against it.