Human immunity to disease and infection is both an active and reactive process. Immune cells and their products protect the body from invading organisms, but also actively seek and destroy any that get past the initial lines of defense. Some act on specific targets and others non-specifically based on cell markers, which are mainly membrane proteins, identifying an organism or cell as self or foreign.
Immune cells are white blood cells, called leukocytes. They are produced in the bone marrow and a gland called the thymus. They circulate in the blood and lymphatic tissues where they undergo a constant evaluation of surrounding materials, colliding and sensing what markers they possess. They are also, to some degree, stored in the lymph nodes and spleen, centers of activity during infections.
There are a number of cells involved in human immunity, many of them being either quite specialized or advanced versions of other types. The most basic types though are phagocytes, granulocytes, and lymphocytes.
Phagocytes are cells that engulf cellular debris and pathogens. They include macrophages, neutrophils, and dendritic cells. Dendritic cells are named for their appearance; they have dendrites, or small arms, that radiate out from their center. These arms present pieces of the material they have engulfed so more specialized cells can turn on and fight any invading agent. This makes dendrites an antigen-presenting cell (APC). Macrophages are also APCs. They act both for initial defense and turning on a response.
Granulocytes are cells that release their granules, which are chemicals they carry with them. This group includes eosinophils and basophils. Their chemicals enzymatically attack pathogens and infected cells. They also promote the inflammatory response. Similar in nature are mast cells. Mast cells are found in connective tissues and mucous membranes. They regulate the inflammatory response and take part in allergic reactions by releasing histamine.
Lymphocytes are specialized leukocytes and more well known as T cells and B cells. These cells recognize cell surface proteins called antigens and take part in the adaptive, or cell-mediated, immune response. Natural killer cells (NK cells) are often considered to be a type of lymphocyte. They attack and destroy infected cells or tumor cells.
T cells are conditioned in the thymus to recognize self-antigen, which are proteins expressed by a person's own cells. T cells distinguish between invading elements and host cells by two different types of cell surface complexes, called MHC complexes. This difference in recognition, class I or II, is one difference between CD4 (helper) and CD8 (cytotoxic) T cells. Cytotoxic T cells release cytotoxins that kill infected or damaged host cells. Helper T cells secrete cytokines, which are more chemicals, to activate other cells in the immune response.
Some lesser known subsets of T cells are memory T cells, which are antigen-specific, and therefore invading-organism-specific, and are part of the long-term immune response. The cells recognize later infections by the same agent and can promote a faster response. Regulatory T cells, also known as suppressor T cells, suppress the immune response to prevent over activity against the host.
B cells are activated in a two step process of engulfing the foreign particles and recognizing them on their own surface. Plasma cells are mature B cells that make antibodies against those specific antigens. Antibodies are proteins that bind to the surface of a cell to mark it for destruction by other components of the immune response (called complement). Memory cells are long-term activated B cells that can quickly begin antibody production against re-infection by the same pathogen. Some antibodies are produced as a first line of defense in the mucus membranes, other specifically to attack an invading particle or organism.