In humans two main types of immunity are recognized: the innate (non-specific) and the adaptive (also known as acquired) immunity. Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules (pathogens).
If a pathogen such as a bacteria or virus manages to breach the layered defenses such as the physical barriers of the skin, the innate immune system provides an immediate, but non-specific response. If the pathogen then successfully evades the innate response, vertebrates possess a second layer of protection called the adaptive immune system, which is activated by the innate response.
The innate immune system is the dominant system of defense in most organisms and it is found in all plants and animals.
The innate immune system is non-specific, its response comes into play immediately or within hours of an antigen’s appearance in the body in a very effective way but does not confer long lasting immunity against such a pathogen since it lacks an immunological memory.
Cells of the innate immune system include phagocytes cells (white blood cells that engulf and absorb waste material, harmful micro organisms or other foreign bodies) such as monocytes and macrophages, and also natural killer (NK) cells, basophils, mast cells, eosinophils and dendritic cells.
Cytokines released from innate immune cells play a key role in the regulation of the immune response.
Cytokines secreted by different innate immune cells includes tumor necrosis factor (TNF), interleukins IL-1, IL-6, IL-10, IL-12, IL-15, and IL-18, interferons such as IFN-α, IFN-β and IFN- γ.
The adaptive (acquired) immune system, on the other hand, allows for a stronger immune response as well as an immunological memory which is maintained by ‘memory cells’.
This highly effective specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells.
Cells that make up the adaptive immune system include the B and T lymphocytes which are derived from hematopoietic stem cells in the bone marrow.
B lymphocytes are involved in the humoral immune response whose primary function is the production of antibodies, whereas T lymphocytes differentiate into different subtypes like the T cytotoxic (Tc) cells,T helper (Th) cells, natural killer T cells (NKT cells).
Cytokines that play a role in the adaptive immune system include interleukins IL-2, IL-4, IL-5, IL-10, IL-12,TGF-β and IFN-γ.