ABSTRACT
The immune system is composed of an adaptive immune system and an innate immune system. The adaptive immune system distinguishes itself from the innate system by the following features: Specificity of antigen recognition, Diversity of the antigen receptor repertoire, Rapid clonal expansion, Adaptiveness to the changing environment, and Immunological memory. The innate system lacks fine specificity, has limited diversity and rudimentary memory but manifests rapid engagement. Lymphocytes are the primary cells of adaptive immunity; they include T cells, B cells and Natural Killer (NK) cells. This chapter describes each individual cell type. Professional Antigen Presenting Cells (APC) are those that express high levels of class II MHC molecules and possess the capacity to internalize, process and present foreign antigens in the MHC groove. APC cells include Dendritic Cells, B cells, monocytes/macrophages and their tissue counterparts. NK cells do not possess the antigen receptor, making it a component of innate immunity.
