The immune system is a complex integrated network of chemical and cellular mediators, biological structures and processes, developed over the course of evolution to defend the body against any form of chemical, traumatic or infectious insult to its integrity. To function properly, an immune system must be able to detect a wide variety of agents, known as pathogens, from viruses to helminths and distinguish them from the body’s own healthy tissue. The function of these components is divided up into nonspecific mechanisms, those which are innate to an organism, and responsive responses, which are adaptive to specific pathogens. Fundamental or classical immunology involves studying the components that make up the innate and adaptive immune system.
Innate immunity is the first line of defence and is non-specific. That is, the responses are the same for all potential pathogens, no matter how different they may be. Innate immunity includes physical barriers (e.g. skin, saliva etc) and cells (e.g. macrophages, neutrophils, basophils, mast cells etc). These components ‘are ready to go’ and protect an organism for the first few days of infection. In some cases, this is enough to clear the pathogen, but in other instances the first defence becomes overwhelmed and a second line of defence kicks in.
Adaptive immunity is the second line of defence which involves building up memory of encountered infections so can mount an enhanced response specific to the pathogen or foreign substance. Adaptive immunity involves antibodies, which generally target foreign pathogens roaming free in the bloodstream. Also involved are T cells, which are directed especially towards pathogens that have colonised cells and can directly kill infected cells or help control the antibody response.
In many species, the immune system can be classified into subsystems, such as the innate immune system opposed to the adaptive immune system or humoral immunity opposed to cell-mediated immunity. In humans, the blood-brain barrier, the blood-cerebrospinal fluid barrier, and other similar fluid-cerebral barriers separate the peripheral immune system from the neuroimmune system that protects the brain.
Pathogens can rapidly evolve and adapt, thereby managing to avoid detection and neutralization by the immune system; however, several defense mechanisms have evolved to overcome this. Even simple single-celled organisms, such as bacteria, possess a rudimentary immune system, consisting of enzymes that protect against infection by bacteriophages. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and invertebrates. These include phagocytosis, antimicrobial peptides called defensins, and the complement system.
Gnathostomes, including humans, boast even more sophisticated defensive mechanisms, including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive immunity creates an immunological memory following an initial response to a specific pathogen, thus leading to a better response to any subsequent encounters with the same pathogen. This process of acquired immunity is the basis of vaccination.
Disorders of the immune system can cause autoimmune diseases, inflammatory diseases, and cancers. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurrent, life-threatening infections. In humans, immunodeficiency can be the result of a genetic disease such as severe combined immunodeficiency, or acquired through diseases such as HIV/AIDS, or through the use of immunosuppressive drugs. In contrast, autoimmunity from an overactive immune system involves attacking normal tissues as if they were foreign organisms. The most common autoimmune diseases are Hashimoto’s thyroiditis, rheumatoid arthritis, type 1 diabetes mellitus, and systemic lupus erythematosus. Immunology involves the study of all aspects of the immune system.
A fundamental characteristic of the immune system is therefore the ability to distinguish between endogenous or exogenous structures that do not pose a danger and therefore can or should be preserved (self) and endogenous or exogenous structures that are harmful to the organism and therefore must be eliminated (non-self).
According to the most recent theories, the immune system distinguishes a non-infectious self from an infectious self. The discrimination between self and non-self occurs at the molecular level and is mediated by particular cellular structures (Toll-like receptor, T lymphocyte receptors, MHC complexes, antibodies), which allow the presentation and recognition of components of the injurious agent called antigens (literally, antibody inducers).
Depending on the mode of recognition of antigens, two areas of the immune system can be distinguished:
- non-specific or innate immunity: includes chemical mediators (responsible for inflammation) and cellular mediators responsible for a first line of defense against aggression. It is evolutionarily older and allows the recognition of a limited repertoire of antigens. It recognizes a generic condition of danger and puts the immune system in a condition of “alarm”, which promotes the development of specific immunity;
- specific or acquired or adaptive immunity: includes chemical and cellular mediators responsible for a more powerful and targeted defensive response (virtually able to recognize any form of antigen), but slower. It is evolutionarily more recent and relies on the non-specific response for numerous functions of presentation and destruction of antigens. It is in turn divided into:
- humoral specific immunity (i.e., antibody-mediated);
- specific cell-mediated immunity.