Innate Immune Response

The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms (see Beyond vertebrates)..

The major functions of the innate immune system are to:
* recruit immune cells to infection sites by producing chemical factors, including chemical mediators called cytokines
* activate the complement cascade to identify bacteria, activate cells, and promote clearance of antibody complexes or dead cells
* identify and remove foreign substances present in organs, tissues, blood and lymph, by specialized white blood cells
* activate the adaptive immune system through antigen presentation
* act as a physical and chemical barrier to infectious agents; via physical measures such as skin and chemical measures such as clotting factors in blood, which are released following a contusion or other injury that breaks through the first-line physical barrier (not to be confused with a second-line physical or chemical barrier, such as the blood–brain barrier, which protects the nervous system from pathogens that have already gained access to the host).

Anatomical barriers

Anatomical barriers include physical, chemical and biological barriers. The epithelial surfaces form a physical barrier that is impermeable to most infectious agents, acting as the first line of defense against invading organisms. Desquamation (shedding) of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surface. Lack of blood vessels, the inability of the epidermis to retain moisture, and the presence of sebaceous glands in the dermis, produces an environment unsuitable for the survival of microbes. In the gastrointestinal and respiratory tract, movement due to peristalsis or cilia, respectively, helps remove infectious agents. Also, mucus traps infectious agents. Gut flora can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or cell surface attachment sites. The flushing action of tears and saliva helps prevent infection of the eyes and mouth.

Inflammation

Inflammation is one of the first responses of the immune system to infection or irritation. Inflammation is stimulated by chemical factors released by injured cells. It establishes a physical barrier against the spread of infection and promotes healing of any damaged tissue following pathogen clearance.

The process of acute inflammation is initiated by cells already present in all tissues, mainly resident macrophages, dendritic cells, histiocytes, Kupffer cells, and mast cells. These cells present receptors contained on the surface or within the cell, named '' pattern recognition receptors'' (PRRs), which recognize molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen-associated molecular patterns (PAMPs). At the onset of an infection, burn, or other injuries, these cells undergo activation (one of their PRRs recognizes a PAMP) and release inflammatory mediators, like cytokines and chemokines, which are responsible for the clinical signs of inflammation. PRR activation and its cellular consequences have been well-characterized as methods of inflammatory cell death, which include pyroptosis, necroptosis, and PANoptosis. These cell death pathways help clear infected or aberrant cells and release cellular contents and inflammatory mediators.

Chemical factors produced during inflammation ( histamine, bradykinin, serotonin, leukotrienes, and prostaglandins) sensitize pain receptors, cause local vasodilation of the blood vessels, and attract phagocytes, especially neutrophils. Neutrophils then trigger other parts of the immune system by releasing factors that summon additional leukocytes and lymphocytes. Cytokines produced by macrophages and other cells of the innate immune system mediate the inflammatory response. These cytokines include TNF, HMGB1, and IL-1.

The inflammatory response is characterized by the following symptoms:
* redness of the skin, due to locally increased blood circulation;
* heat, either increased local temperature, such as a warm feeling around a localized infection, or a systemic fever;
* swelling of affected tissues, such as the upper throat during the common cold or joints affected by rheumatoid arthritis;
* increased production of mucus, which can cause symptoms like a runny nose or a productive cough;
* pain, either local pain, such as painful joints or a sore throat, or affecting the whole body, such as body aches; and
* possible dysfunction of involved organs/tissues.

Complement system

The complement system is a biochemical cascade of the immune system that helps, or “complements”, the ability of antibodies to clear pathogens or mark them for destruction by other cells. The cascade is composed of many plasma proteins, synthesized in the liver, primarily by hepatocytes. The proteins work together to:
* trigger the recruitment of inflammatory cells
* "tag" pathogens for destruction by other cells by ''opsonizing'', or coating, the surface of the pathogen
* form holes in the plasma membrane of the pathogen, resulting in cytolysis of the pathogen cell, causing its death
* rid the body of neutralised antigen-antibody complexes.
The three different complement systems are classical, alternative and lectin.
* Classical: starts when antibody binds to bacteria
* Alternative: starts "spontaneously"
* Lectin: starts when lectins bind to mannose on bacteria

Elements of the complement cascade can be found in many non-mammalian species including plants, birds, fish, and some species of invertebrates.

White blood cells

White blood cells (WBCs) are also known as leukocytes. Most leukocytes differ from other cells of the body in that they are not tightly associated with a particular organ or tissue; thus, their function is similar to that of independent, single-cell organisms. Most leukocytes are able to move freely and interact with and capture cellular debris, foreign particles, and invading microorganisms (although macrophages, mast cells, and dendritic cells are less mobile). Unlike many other cells, most innate immune leukocytes cannot divide or reproduce on their own, but are the products of multipotent hematopoietic stem cells present in bone marrow.

The innate leukocytes include: natural killer cells, mast cells, eosinophils, basophils; and the