The immune system is designed to protect the host from a wide range of pathogens and exogenous threats while distinguishing self from non-self. This overview highlights the key mechanisms of the immune response, including both innate and adaptive immunity, and discusses the importance of self-tolerance to prevent damage to host tissues. The innate immune system, encoded by germ-line genes, includes physical barriers, soluble proteins, and membrane-bound receptors that recognize molecular patterns shared by pathogens and toxins. The adaptive immune system, which is highly specific, involves the somatic rearrangement of gene elements to form antigen-binding molecules with exquisite specificity. The T cell arm of the immune system plays a crucial role in recognizing infected host cells by requiring the recognition of both self-antigens and microbial structures. Major histocompatibility complex (MHC) molecules, such as class I and class II MHC molecules, are essential for this process. The presentation of antigens by MHC molecules to T cells is a critical step in the immune response, and the diversity of MHC molecules ensures broad antigen recognition. Additionally, the HLA types are associated with disease susceptibility, and some diseases show linkage to specific HLA alleles. The immune system also includes a variety of leukocyte subsets, such as B cells, T cells, natural killer (NK) cells, and myeloid cells, each with distinct functions in immune responses. The development of T cells involves positive and negative selection in the thymus, leading to the formation of functional TCRs and the acquisition of effector functions. T cells can differentiate into various subsets, such as Th1, Th2, and Th17, each with distinct cytokine profiles and effector functions. Superantigens are microbial products that bind to large subsets of TCRs and MHC molecules, leading to a strong immune response.The immune system is designed to protect the host from a wide range of pathogens and exogenous threats while distinguishing self from non-self. This overview highlights the key mechanisms of the immune response, including both innate and adaptive immunity, and discusses the importance of self-tolerance to prevent damage to host tissues. The innate immune system, encoded by germ-line genes, includes physical barriers, soluble proteins, and membrane-bound receptors that recognize molecular patterns shared by pathogens and toxins. The adaptive immune system, which is highly specific, involves the somatic rearrangement of gene elements to form antigen-binding molecules with exquisite specificity. The T cell arm of the immune system plays a crucial role in recognizing infected host cells by requiring the recognition of both self-antigens and microbial structures. Major histocompatibility complex (MHC) molecules, such as class I and class II MHC molecules, are essential for this process. The presentation of antigens by MHC molecules to T cells is a critical step in the immune response, and the diversity of MHC molecules ensures broad antigen recognition. Additionally, the HLA types are associated with disease susceptibility, and some diseases show linkage to specific HLA alleles. The immune system also includes a variety of leukocyte subsets, such as B cells, T cells, natural killer (NK) cells, and myeloid cells, each with distinct functions in immune responses. The development of T cells involves positive and negative selection in the thymus, leading to the formation of functional TCRs and the acquisition of effector functions. T cells can differentiate into various subsets, such as Th1, Th2, and Th17, each with distinct cytokine profiles and effector functions. Superantigens are microbial products that bind to large subsets of TCRs and MHC molecules, leading to a strong immune response.