Human blood contains numerous erythrocytes (red blood cells) and several kinds of leukocytes (white blood cells) that play important roles in the internal immunity system.

Basophils release histamine and can cause allergic reactions.

Eosinophils discharge destructive enzymes against parasitic worms.

Lymphocytes recognize and respond to foreign molecules (antigens).

Neutrophils and monocytes (macrophages) are phagocytes.