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 by antigen-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 cells and cytotoxic (or killer) T cells.