Phagocytosis means “big eater” while the phagocytosis process involves a cell engulfing a particle, digesting the particle, and then expelling the waste products. Phagocyte cells protect the body by phagocytosing or ingesting pathogenic or harmful non-self foreign particles, bacteria, fungi and dead or dying cells.

Phagocytes include neutrophils, monocytes, macrophages, dendritic cells and mast cells. These phagocytes all have surface receptors by which they can detect pathogens, especially bacteria, viruses, fungi and even parasites.  The steps in the process of phagocytosis include: 1. particle ingestion by a phagocyte after antigen recognition triggers the formation  of a phagosome. 2. phagosomes fuse with lysosomes to creates a phagolysosome particle. 3. digestive enzymes from the lysosomes break down the phagolysosome particle, with the waste exiting the cell by exocytosis.

As an example of an immune response, activated macrophages ingest a body invader such as a virus and display pieces of the virus known as antigens on the macrophage surface.  The helper T-Cells recognize specific antigens on the macrophage surface (apc cells) and bind to the macrophages.  The helper T-cells then evaluate the situation and call in the various immune system components needed to fight back effectively against a specific antigen.

The helper T-cells also activate the humoral side of the immune response, including the B-cells. B-lymphocytes develop and mature in the bone marrow and on presentation of a foreign antigen (non-self) on invasion of the body, begin to divide and secrete a protein known as an antibody.  These unique antibodies then bind to the antigens on the surface of the health invaders, enabling recognition by the macrophages and various T-cells classified as Cytotoxic T-cells that kill with chemicals and NK or Natural Killer T-cells that kill by lysing or ramming the pathogens.

T-lymphocytes or T-cells are part of the cellular immune response. These lymphocytes are also produced in the bone marrow where they then migrate to the thymus to become mature T-cells. On the surface of the T-cell is an antigen-binding molecule known as the T-cell receptor.  The T-cell can bind antigen, but it must be in the presence of a complex of proteins on the cells of the host, called the major histocompatibility complex (MHC). T-lymphocytes are mainly involved in the response to altered cells of the host, as in those infected by a virus or those that develop into cancer cells.

This antigen binding also alerts blood components known as complements (actually 25 different proteins) to puncture holes in invader infected cells.  This facilitates phagocytosis, or ingestion of the invader by phagocytes such as the macrophage.

As the health invader is killed and brought under control, the activated T-cells and B-cells are signaled to stop attacking by suppressor T-cells.  Another type of T-cell, the Memory T-cells, remain behind to create acquired immunity and to respond quickly against the same type of invader, if the same health invader attempts to attack again.  Your body contains millions of antibodies at any given time. We often translate this to mean we are building up an immunity to various viruses, bacteria, fungi or parasites. Vaccines present a very small quantity of a specific antigen for which the B-cells produce antibodies unique to the specific antigen that are then available as memory cells to respond quickly if a future attack occurs as in influenza or polio.