Figure 1.

The human immune system and its functions. (A) The immune system consists of organs (thymus, tonsils, spleen, lymph, lymphatic tissue of the gut and bone marrow), immune cells (T cells, NK cells, B cells, macrophage, etc.), and molecules (antibodies, cytokines, chemokines, etc.). It protects against foreign pathogens, eliminates cancer cells, and maintains normal body functions. (B) Two primary arms of effector immunity are innate and adaptive immunities for immune response. Innate immunity is nonspecific and refers to defense mechanisms activated rapidly to prevent the spread of foreign antigens, including those produced by cancer cells. The main cell types involved in innate immunity are natural killer cells (NK), dendritic cells (DC), and macrophages. NK cells are activated by cancers through NK cell activating receptor interaction with its ligands in cancer cells, releasing perforin and other killer molecules to trigger cancer cell death. Adaptive (acquired) immunity comprises the second line of defense, responding to particular ‘non-self’ antigens expressed in cancer cells. It is characterized by clonal expansion of T and B lymphocytes (T cell and B cell); upon expansion, these clonal cells express the same antigen receptor and are primed to fight the same pathogen. B lymphocytes are primarily involved in antibody-mediated immunity, while T lymphocytes are mainly associated with cell-mediated immunity. After activation through interaction, T cells and tumor antigens represented by DC, CD8+ T cells become cytotoxicity T cells, and CD4+ T cells release cytokines to stimulate CD8+ T cells to convert to cytotoxicity T cells. Cytotoxicity T cells recognize the cancer cells and mediate cancer cell clearance through direct cytotoxicity or secretion of inflammatory cytokines.