T-cells and B-cells are derived from stem cells that originate in the bone marrow. B-cells continue to mature in the bone marrow; however, T-cells mature in the thymus. Once they are mature, they travel through the blood and reside in the lymph.

Cell-mediated immunity involves all of the differentiated forms of T-lymphocytes. Cytotoxic T-cells (also called killer T-cells) are antigen-specific, and responsible for the destruction of infected cells bearing the given antigen.

Plasma cells and B-cells are part of humoral (antibody) immunity, while neutrophils play an important role in the innate immune response.

When an antigen is presented to a B-cell, it can develop the ability to produce antibodies against that specific antigen. Once the B-cell gains this ability, it is a plasma cell.

T-cells, B-cells, and dendritic cells are derived from lymphoid progenitor stem cells. Together, the group of cells derived from this cell type is known as the lymphocytes. Mast cells, erythrocytes, and monocytes are derived from myeloid progenitor stem cells. Macrophages are derived from monocytes.

Blood cells can be broken into two main categories based on their originating stem cells. Myeloid progenitor cells are the source of erythrocytes and most granulocytes. Lymphoid progenitor cells give rise to lymphocytes.

Lymphocytes are mostly involved in the adaptive immune response, and include dendritic cells, T-cells, B-cells, and the derivatives of these cells, such as plasma cells and memory cells. Natural killer cells, however, are granulocytes that are involved in the innate immune response, but are still derived from lymphoid progenitor cells. Natural killer cells are thus lymphocytes as well.

Neutrophils are granulocytes involved in the innate immune response, and are generated from myeloid progenitor cells; neutrophils are not lymphocytes.

The thymus is responsible for the maturation of T-cells. The thymus, a gland centered in the chest, allows T-cells to be tested against a "self-antigen." If the T-cell does not react against the self-antigen, it is allowed to leave the thymus and enter into circulation to detect foreign antigens. If the T-cell does react to the self-antigen, it is generally destroyed. Failure to destroy these T-cells can result in autoimmune attacks. Upon detection of foreign antigens, mature T-cells differentiate into a variety of other T-cell types and stimulate B-cells to produce antibodies.

Eosinophils primarily mediate reactions to parasitic infections, including worms. Neutrophils respond to bacterial infections, and basophils to allergies (IgE). Megakaryocytes are responsible for making platelets.

Basophils and mast cells are responsible for secreting factors like histamine in response to allergens and drugs. Release of the histamine can cause throat constriction, hives, and inflammation. Eosinophils mediate reactions to parasitic infections and neutrophils to bacterial infections. Megakaryocytes produce platelets.

Neutrophils contain antimicrobial products in vesicles, which are secreted when they come into contact with foreign cells. Neutrophils primarily mediate reaction to bacterial infections and are the primary component of the white pus found in bacterial inflammation. Eosinophils respond to parasitic infections and basophils to allergies (IgE). Megakaryocytes produce platelets.

The granulocytes that participate in innate immunity have vesicles filled with chemicals and proteins, such as perforin and trypsin, that are released to kill foreign bacteria, fungi, and parasites. The granulocytes include neutrophils, eosinophils, basophils, and mast cells.

Neutrophils are involved in bacterial infections and eosinophils in parasitic infections. Basophils and mast cells are involved in allergic reactions and inflammation.

Macrophages are responsible for ingesting and degrading bacteria and viruses, and presenting their antigens on major histocompatibility complexes (MHCs) to B- and T-cells. Macrophages are thus responsible for mediating the initiation of an immune reaction.

Megakaryocytes produce platelets, and B- and T-cells are the lymphocytes to which macrophages present ingested antigens.