Cells of the Immune System
The cells of the immune system are principally the white blood cells or leukocytes. These cells are derived from hematopoietic stem cells that originate in the bone marrow.
Granulocytes: have prominent cytoplasmic granules containing reactive substances that kill microorganisms and stimulate other divisions of the immune response by activating inflammation.
- Neutrophils (also known as polymorphonuclear leukocytes or PMNs because of their multilobed nucleus) are the most numerous white blood cells in the blood. They are particularly good at phagocytosis (the process of capturing, engulfing, and killing microorganisms). Neutrophils are rapidly recruited to sites of inflammation.
- Eosinophils are granulocytes that are important for defenses against helminth worms and other intestinal parasites. They also play a role in allergies (hypersensitivity reactions) and asthma.
- Basophils also participate in the defense against parasites, although that process is less well understood.
Monocytes: are leukocytes that circulate in the blood; as they migrate into the tissue, they become resident tissue macrophages. Macrophages are phagocytic cells that rid the body of dead cells, cellular debris, and serve as sentinel cells for the early detection of infection and tissue damage.
They also produce cytokines that activate inflammation, recruit leukocytes such as neutrophils and additional macrophages to the site of the infection, and assist in activating the adaptive immune response by serving as antigen-presenting cells for T cells.
Dendritic cells: are branched or dendritic-shaped cells that are very effective for initiating T-cell responses. Dendritic cells are resident within the tissues of the body, where they are phagocytic. They then enter the lymphatics to travel to the local secondary lymph tissue, where they express additional adhesins for interaction with T cells and present the antigens from the peripheral tissues.
Mast cells: are resident in all connective tissues. They possess granules that contain a number of preformed inflammatory mediators, including histamine and tumor necrosis factor (TNF)-a. Together with IgE, they are involved in type I hypersensitivity reactions.
Natural killer (NK) cells: are large, granular, lymphocyte-like cells that function in the targeted killing of cells such as virus-infected cells and tumor cells.
Lymphocytes: are the primary cells of the adaptive immune response. All B and T lymphocytes are antigen specific via antigen receptors on the cell surface. Lymphocyte antigen receptors structurally consist of globular protein motifs know as immunoglobulin (Ig) domains. Ig domains are characteristic of many cell surface and secreted proteins of the immune system. In this chapter, the terms B cell and T cell are used instead of B lymphocyte and T lymphocyte.
- B lymphocytes (B cells) are primarily responsible for humoral immunity (antibody response).
- T lymphocytes (T cells) are responsible for cell-mediated acquired immunity.
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