Innate immunity is the first line of defense against invading pathogens and consists of physical, chemical, and biological barriers. These barriers include skin, mucous membranes, cilia, stomach acid, enzymes in tears and saliva, and the inflammatory response. If these innate defenses fail, adaptive immunity comes into play.
Adaptive immunity is a more sophisticated immune response that is specific to a particular pathogen. It includes both humoral immunity, which involves antibodies produced by B cells, and cell-mediated immunity, which involves T cells. The adaptive immune response takes longer to mount than the innate immune response but provides lifelong protection against re-infection with the same pathogen.
The human immune system is constantly vigilant, searching for signs of infection or other threats to homeostasis. When it detects a foreign invader or abnormal cell growth (such as cancer), it mounts an immune response to eliminate the threat. The Immune System is composed of many types of cells that work together to protect the body:
# Antigen-presenting cells (APCs) “present” pieces of foreign material (antigens) on their surface so that other cells of the immune system can recognize them as being different from “self”
# B lymphocytes produce antibodies that bind to antigens
# T lymphocytes directly kill infected host cells or help activate B lymphocytes
# Natural killer (NK) cells destroy virally infected or cancerous cells
# Phagocytes engulf and digest foreign particles and microorganisms
Cells of the Immune System are able to distinguish between self and non-self because they have receptors on their surface that can bind specifically to proteins called antigens. Antigens are found on bacteria, viruses, fungi, parasites – anything that could potentially harm us if left unchecked. Once an antigen binds to its receptor on an Immune Cell , it sets off a series of events inside the cell that eventually leads to destruction of the microbe or removal of the offending tissue .