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For many years, immunology remained an empirical subject in which the effects of injecting various substances into hosts were studied primarily in terms of the products elicited. Most progress came in the form of more quantitative methods for detecting these products of the immune response. A major change in emphasis came in the 1950s with the recognition that lymphocytes were the major cellular players in the immune response, and the field of cellular immunology came to life.

A convenient way to define the cell types involved in adaptive immunity is to divide the host defense mechanisms into two categories, namely B‐cell and T‐cell responses. While this is an oversimplified definition, it is, by and large, the functional outcome of adaptive immune responses. Thus, defining the cells involved begins with a short list, namely B and T cells. These cells are derived from a common lymphoid precursor cell but differentiate along different developmental lines, as discussed in detail in Chapters 8–10. In short, B cells develop and mature in the bone marrow whereas T‐cell precursors emerge from the bone marrow and undergo critical maturation steps in the thymus.

Antigen‐presenting cells, such as macrophages and dendritic cells, constitute the third cell type that participates in the adaptive immune response. Although these cells do not have antigen‐specific receptors as do the lymphocytes, they process and present antigen to the antigen‐specific receptors expressed by T cells. The APCs express a variety of cell‐surface molecules that facilitate their ability to interact with T cells. Among these are the major histocompatibility complex (MHC) molecules as discussed in Chapter 8. MHC molecules are encoded by a set of polymorphic genes expressed within a population. While we now understand that their physiological role is concerned with T cell–APC interactions, in clinical settings, MHC molecules determine the success or failure of organ and tissue transplantation. In fact, this observation facilitated their discovery and the current terminology (major histocompatibility complex) used to define these molecules. Physiologically, APCs process protein antigens intracellularly, resulting in the constellation of peptides that noncovalently bind to MHC molecules and ultimately get displayed on the cell surface.

Other cell types, such as neutrophils and mast cells, also participate in adaptive immune responses. In fact, they participate in both innate and adaptive immunity. While these cells have no specific antigen recognition properties and can be activated by a variety of substances, they are an integral part of the network of cells that participate in host defenses and often display potent immunoregulatory properties.