Phagocytes also include dendritic cells in tissues that are in contact with the external environment. Thus, they are located mainly in the skin, nose, lungs, stomach, and intestines. Dendritic cells, with their spine-like projections, serve as a link between bodily tissues and the innate and adaptive immune systems. They present antigens to T cells, one of the key cell types of the adaptive immune system.
Other leukocyte cells involved in the innate response are granulocytes (innate lymphoid cells, mast cells, eosinophils, basophils, and natural killer (NK) cells) that have granules in their cytoplasm. Innate lymphoid cells (ILCs) are a group of innate immune cells that are derived from common lymphoid progenitor and belong to the lymphoid lineage. Mast cells reside in connective tissues and mucus membranes, and regulate the inflammatory response. They are most often associated with allergy and anaphylaxis. Basophils and eosinophils are related to neutrophils. They secrete chemical mediators that are involved in defending against parasites and play a role in allergic reactions, such as asthma.
The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules (by a signature antigen) it has previous encountered. Thus, if it returns, it can quickly be eliminated. The ability to mount these tailored responses is maintained in the body by ‘memory cells.’ These cells are special types of leukocytes called lymphocytes, B cells, and T cells that are derived from hematopoietic stem cells in the bone marrow. B cells are involved in the humoral immune response. T cells are involved in cell-mediated immune response. Killer T cells, helper T cells, and regulatory T cells each recognize different antigens in their different roles.
The double-positive T cells are exposed to a wide variety of self-antigens in the thymus, in which iodine is necessary for its thymus development and activity.
Though different roles, each of these cells and their systems work in unison to protect the body and organs they live in.
In contrast, the B cell antigen-specific receptor is an antibody molecule on the B cell surface and recognizes native (unprocessed) antigens without any need for antigen processing. Each lineage of B cell expresses a different antibody. The complete set of B cell antigen receptors represent all the antibodies that the body can manufacture. When B or T cells encounter their related antigens they multiply and many ‘clones’ of the cells are produced that target the same antigen.
There are two types of subtypes of T cells: the killer T cell and the helper T cell. Plus, there are regulatory T cells, which modulate immune response. Killer T cells kill cells that are infected with viruses and other pathogens or are otherwise damaged or dysfunctional. Killer T cells travel throughout the body in search of cells where the MHC 1 receptors bear this antigen. When activated, it releases cytotoxins, such as perforin, which then, in a process causes the cell to undergo apoptosis (destruction).
Helper T cells regulate both the innate and adaptive immune responses and help determine which responses the body makes to a particular pathogen. The activation of resting helper T cell causes it to release cytokines that influence the activity of many cell types. Cytokines signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells. This activation also provides stimulatory signals to activate antibody producing B cells.