Immunology for dummies—a painless review of basic concepts (Proceedings)
The immune system has two arms of defense, nonspecific and specific. As with the immune system in total, there are many elements of the nonspecific arm of the immune response. These have in common their lack of strict recognition of foreign material and absence of memory. The nonspecific arm is also referred to as innate immunity, as it is present at birth unlike specific acquired immunity, which develops after birth and is also known as adaptive immunity. The innate response is the first line of defense against infectious disease, and if effective, may completely eliminate the agent before the specific adaptive immune response is called upon. It also interacts with the adaptive immune response, aiding its activation and modulating the response.
The simplest components of the innate response to understand are the anatomic and physiologic barriers. These include the skin and mucous membranes, as well as physical parameters like temperature, pH, and oxygen levels. These barriers are comprised not only of the cells at the surface, but soluble components as well, such as enzymes, antimicrobial peptides, and cytokines.
More complex, but still nonspecific in terms of antigen recognition are the phagocytic and cytotoxic cells, as well as various soluble components. Cellular components include neutrophils, eosinophils, basophils, mast cells, and natural killer lymphocytes. The first four types are of granulocyte lineage, and possess cytosolic granules filled with enzymes and microbicidal substances; in addition, neutrophils and eosinophils are phagocytic. Natural killer cells, though from the lymphocyte lineage, lack antigen-specific receptors on their surface, and are important mediators of cell-mediated immunity. Distinct from these cells are the monocytes/macrophages, and dendritic cells. Monocytes are phagocytic cells circulating in the blood, while macrophage is the term for these cells in tissue. Dendritic cells are distinct phagocytic cells found in tissue and lymphoid tissue. All three of these cell types, in addition to phagocytosis and destruction of invading microbes, are critical to the specific immune response. In particular, dendritic cells are integral to an effective and appropriate immune response.The specific immune response involves lymphocytes, the only cells which possess specificity, diversity of recognition, and memory. The lymphocytes can be divided into subgroups based on cell surface markers, as well as function. The B lymphocytes are the antibody factories of the immune response (secretors are termed plasma cells), and also function as antigen presenting cells for the T helper lymphocytes. This latter group can be considered the orchestrators of the specific response, and carry out this mission through secretion of various cytokines, soluble messengers of the immune response. This group can be subdivided further based on the direction in which the T helper cell pushes the response, and are designated by subtype numbers of descriptions, i.e. TH1, TH2, TH17, and T regulatory lymphocytes. A group of T lymphocytes distinct from T helper lymphocytes is the cytotoxic T lymphocytes, the hired assassins. They specifically target cells that have been altered, such as by microbial invasion, and induce apoptosis in the target cell, in essence removing the "microbial factory".
While the innate immune response is not specific in terms of antigen recognition, many components do have pattern recognition. Microbial agents contain unique molecules not found in higher organisms, referred to as pathogen associated molecular patterns (PAMP's). The receptors for these molecules, referred to as pathogen recognition receptors (PRRs), may be soluble in plasma, present on cell surfaces, or expressed within the cellular cytoplasm. Recognition and binding of microbes by receptors present on cell surfaces lead to activation of intracellular signaling pathways, altering gene expression, and facilitating elimination of the pathogen. The phagocytic cells, including macrophages, dendritic cells, and neutrophils, express a variety of these PRRs, allowing recognition of pathogens, and enhancing phagocytosis and destruction of the pathogen. Dendritic cells in particular are critical to the adaptive immune response, presenting antigen to cytotoxic T lymphocytes as well as T helpers.