The adaptive immune system primarily relies on the function of B-cells and T-cells. T-cells help recognize antigens to which the body has been previously exposed and stimulate B-cell to release antibodies to combat that specific antigen.

Upon initial antigen exposure, all other cell types listed (which help comprise the innate immune system) kick in.

Basophils are the least common leukocyte found in the body, but play a key role in the inflammatory response. They contain histamine, which is a potent vasodilator. Upon release, histamine will increase blood flow to infected areas. Mast cells are another immune cell that is involved in histamine release, but are generally localized to various regions of the body rather than found in circulation.

Basophils, mast cells, eosinophils, and neutrophils are all considered granulocytes and are essential cells in the innate immune response. Plasma cells are differentiated B-lymphocytes that are responsible for mass-producing antibodies to a specific antigen.

Memory B cells circulate the body and are ready to respond to subsequent infections while plasma cells generate many antibodies to a current infect. "Immunoglobulins" refers to both the B cell receptor and to the excreted form of these proteins (known as antibodies). IgG is a class of immunoglobulins (along with IgA, IgE, IgD, and IgM). Finally, macrophages are cells that engulf non-self (and often antibody-bound) cells and communicate with T cells to promote B cell proliferation.

Mast cells contains secretory granules, rich in histamine and other hormonal mediators, that promote inflammation and other allergy symptoms in response to antigen exposure.

The humoral response refers to the antiquated term "humors", meaning body fluids, as used in ancient and medieval medicine. In response to antigen exposure, B-cells release antibodies into the extracellular fluid and throughout the body, thus eliciting a "humoral response."

Although helper T-cells are responsible for activating B-cells, the humoral response is limited to B-cells because they are the ones releasing products into the body fluids.

Red blood cells are made in the bone marrow, and live for about 3-4 months. They are enucleated (lacking a nucleus), which makes more space available for hemoglobin molecules, which function to carry oxygen to the tissues. HIV infects T-helper cells, which are white blood cells, not red blood cells. Thus, the name makes sense since the virus infects cells of the immune system (white blood cells) and causes immunodeficiency. The liver and spleen play roles in recycling the red blood cells once they have carried out their function for about 120 days.

The granulocytes are responsible for numerous functions of innate immunity, from secreting histamine, to phagocytosis, to anti-inflammatory processes. These cells are the basophils, neutrophils, eosinophils, macrophages (monocytes), and mast cells.

Eosinophils have a more limited role in innate defense than the other granulocytes. They possess only low phagocytic activity, however, they are more specialized to respond to multi-cellular pathogens, such as parasitic worms. Rather than phagocytosing an invading organism, eosinophils function by releasing an arsenal of destructive enzymes and free radicals to ward off the organism. The other granulocytes are specialized for phagocytosis of bacteria, viruses, and cellular debris.

Each B-cell receptor is a Y-shape molecule that, upon maturation, can become a secretory form (an antibody). Each receptor is comprised of four polypeptide chains: two identical heavy chains and two identical light chains. The four chains are linked together by disulfide bridges. Both heavy and light chains consist of constant regions, as well as variable regions. The variable regions of each chain provide the specificity for antigen binding, which generates a signaling cascade within the B-cell.

Humoral, or B-cell, immunity is associated with the formation of antibodies. Plasma cells are B-lymphocytes that have been differentiated with the help of a helper T-cell. They release antibodies, which are created to respond to a specific pathogen in the body.

Cytotoxic T-cells are also activates by help T-cells, but are involved in cell-mediated immunity rather than humoral immunity. They target infected cells based on antibody tagging. Monocytes are a part of the innate immune response and are not involved in antibody interactions. They primarily differentiate into macrophages, which engage in phagocytosis of pathogens.

Keratinocytes are not immune cells. Rather, they secrete a protein called keratin which is a large proportion of the extracellular matrix and makes up hair, nails, skin, and other parts of the body. All other cells are immune cells.