Strategy of the Immune System: (a) Immune system is a collection of cells and molecules, (b) Clonal selection and antigen recognition, (c) Cells that process and present antigen with MHC molecules, (d) Activation of lymphocytes, (e) Regulation of the immune response, (f) Attributes of earlier concepts and contribution from newer molecular techniques. Cells of the immune system are composed of (i) phagocytes of innate or natural immunity, (ii) accessory cells such as antigen presenting dendritic cells and (iii) lymphocytes of acquired or adaptive immunity, such as CD19+ or CD20+B cells, CD3 positive T cells and subset of T cells, e.g. CD8+cytotoxic-suppressor and CD4+helper-inducer that induce CD4+Th1 and CD4+Th2 cells, together with some other cell types, e.g. CD4+CD25+ regulatory T cells, natural killer cells or large granular lymphocytes. CD34+stem cells are multi-potent progenitor cells that originate primarily from the liver in the developing foetus and then migrate as it grows to the bone marrow. These cells differentiate into various types of blood cells under the influence of growth factors or cytokines. The lymphocytes separate into B cell and T cell precursors. The B cell precursors mature in the bone marrow and then enter the blood and settle in the secondary lymphoid tissues, whereas the T cell precursors pass through the thymus. While the early pre B lymphocytes develop further and then migrate to the peripheral circulation and spread throughout the body, the pre T lymphocytes learn MHC restriction in the thymus before they become mature and migrate to the periphery and recirculate along with B lymphocytes between blood and lymphoid tissues. MHC restriction is a phenomenon of linked recognition, in which the interaction between helper T cell and antigen presenting cell is MHC class II restricted. This chapter also contains: (a) Some description on gene rearrangement in antigen independent gene reshuffling and genetic reprogramming in antigen dependent process. (b) Concept of immune recognition. (c) BCR and TCR gene organization or VDJ organization. (d) Cells that process and present antigen with MHC molecules. (e) Recognition of antigen and activation of anti-non-self lymphocytes. (f) Regulation of the immune response.