|Antigen-presenting cells |Cells which do not have antigen-specific receptors. Instead, they capture and | | |process antigens, present them to T cell receptors. These cells include | | |macrophages, dentritic cells and B cells. | |B cells |Also known as B cell lymphocytes. | | | | | |B cells spend their entire early life in the bone marrow. Upon maturity, their | | |job is to travel throughout the blood and lymph looking for antigens with which | | |they can interlock. | | | | | |Once a B cell has identified an antigen, it starts replicating itself.
These | | |cloned cells mature into antibody-manufacturing plasma cells. | |Basophils |Similar to mast cells, but distributed throughout the body. Like mast cells, | | |basophils release histamine upon encountering certain antigens, thereby | | |triggering an allergic reaction. | |Cytotoxic T cells |Also called cytotoxic T lymphocytes or CTLs. | | | | |Dendritic cells |Mostly found in the skin and mucosal epithelium, where they are referred to as | | |Langerhan’s cells. Unlike macrophages, dendritic cells can also recognize viral | | |particles as non-self.
In addition, they can present antigens via both MHC I and| | |MHC II, and can thus activate both CD8 and CD4 T cells, directly. | |Granulocytes |Leukocytes (white blood cells) containing granules in the cytoplasm. Also known | | |as a granular leukocyte. They seem to act as a first line of defense, as they | | |rush toward an infected area and engulf the offending microbes. Granulocytes | | |kill microbes by digesting them with killer enzymes contained in small units | | |called lysosomes. | |Helper T cells |These cells travel through the blood and lymph, looking for antigens (such as | | |those captured byantigen-presenting cells).
Upon locating an antigen, they | | |notify other cells to assist in combating the invader. | | | | | |This is sometimes done through the use of cytokines(or specifically, lymphokines)| | |which help destroy target cells and stimulate the production of healthy new | | |tissue. Interferon is an example of such a cytokine. | |Leukocytes |White blood cells. These are the cells which provide immunity, and they can be | | |subdivided into three classes: lymphocytes, granulocytes and monocytes | |Lymphocytes |Small white blood cells which are responsible for much of the work of the immune | | |system.
Lymphocytes can be divided into three classes: B cells, T cells and null| | |cells. | |Macrophages |Literally, “large eaters. ” These are large, long-lived phagocytes which capture | | |foreign cells, digest them, and present protein fragments (peptides) from these | | |cells and manifest them on their exterior. In this manner, they present the | | |antigens to the T cells. | | | | | |Macrophages are strategically located in lymphoid tissues, connective tissues and| | |body cavities, where they are likely to encounter antigens. They also act as | | |effector cells in cell-mediated immunity.
| |Mast cells |Cells concentrated within the respiratory and gastrointestinal tracts, and within| | |the deep layers of the skin. These cells release histamine upon encountering | | |certain antigens, thereby triggering an allergic reaction. | |Memory cells |Specialized B cells which grant the body the ability to manufacture more of a | | |particular antibody as needed, in case a particular antigen is ever encountered | | |again. | |Monocytes |Large, agranular leukocytes with relatively small, eccentric, oval or | | |kidney-shaped nuclei.
| |Plasma cells |Specialized B cells which churn out antibodies—more than two thousand per | | |second. Most of these die after four to five days; however, a few survive to | | |become memory cells. | |T cells |Also known as T cell lymphocytes. | | | | | |Unlike B cells, these cells leave the marrow at an early age and travel to the | | |thymus, where they mature. Here they are imprinted with critical information for| | |recognizing “self” and “non-self” substances. | | | | | |Among the subclasses of T cells are helper T cellsand cytotoxic (or killer) T | | |cells. |