Our innate immune cells and digestion are backed by a very sophisticated defence infrastructure called the adaptive immune system. This is more complex and also more likely to go wrong.
The adaptive (or specific) immune system is formed of type of white blood cells called lymphocytes. These include B cells (or ‘plasma cells’), T cells and Natural Killer (NK) cells. They are responsible for mounting responses to specific antigens.
Unlike the immediately active innate immune system, the adaptive response to a new antigen encounter can take days. However on the basis of this encounter B- and T cells produce ‘memory cells’ which can respond quickly to repeat exposures (memory cells are the aim of vaccination).
B cells have two major functions: they produce antibodies to neutralize antigens and they also present these antigens to T cells. In an infection antibodies can coat germ walls and initiate one of three measures. Neutralization prevents the pathogen from binding and infecting host cells. In opsonization, an antibody-bound pathogen serves as a red flag to alert immune cells like neutrophils and macrophages, to engulf and digest the pathogen. Complement is a process using specific proteins like a limpet mine to directly destroy bacteria.
The most common type of antibody (‘immunoglobulin’ or Ig) produced by B cells are IgG antibodies; other important types are associated with the gut wall: IgA, IgD and IgM. There are also IgE antibodies most associated with basophils ‘(Mast cells’): these are designed to target large parasites like amoebae and worms, but are mostly associated in modern times with allergic responses.
T cells have roles that include killing infected cells and activating or recruiting other immune cells. CD8+ T cells recognize and remove virus-infected cells and cancer cells. The CD4+ T-cells (‘T helper cells’) are responsible for coordinating immune resposes to bacteria and other pathogens. Regulatory T cells (Tregs) monitor and inhibit the activity of other T cells. They prevent adverse immune activation and maintain ‘tolerance’, the prevention of immune responses against the body’s own cells and antigens.
Natural Killer cells are important for recognizing and killing virus-infected or tumour cells by applying safe self-destructive methods (to reduce antigen production in cell death – see earlier post). NK cells can also be retained as memory cells.