Innate and acquired immunity

• The cells of the immune system are divided into lymphoid and myeloid lineages. The former include T lymphocytes and their subsets identified by CD markers, B lymphocytes and natural killer (NK) cells. The myeloid lineage includes the neutrophils, eosinophils and basophils as well as the monocyte/ macrophage series and platelets. • Innate immunity depends on physical, physiological and chemical barriers to infection, on the response to injury and on detection of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs). Phagocytic cells and the enzyme cascade known as complement are key effectors responding to PAMPs and components of acute inflammation. • Acquired immunity depends on specific recognition of antigens either directly by antibodies on the surface of B cells or through presentation of processed antigens in the context of MHC molecules by host cells to T cells. In contrast to innate immunity, on re-exposure the responses are faster, more vigorous and more specific. • Acquired immune responses are driven by the availability of antigen. As they mature, only cells with high-affinity receptors for the antigen are stimulated to divide. The expanded clones of antigen-specific cells are said to have resulted from clonal selection. • Lymphocytes are activated by antigen and the appropriate combination of cytokines, signalling molecules secreted by other lymphocytes and by macrophages. • Humoral acquired immunity leads to antigen-antibody complexes that neutralize key aspects of microbial activity either directly or through the activation of complement, opsonization and directed cytotoxicity. • Cell-mediated immunity generates cytotoxic T lymphocytes (CD8), which directly kill cells containing intracellular pathogens, and helper T cells (CD4), which secrete lymphokines that stimulate other effector aspects of immunity. • Inherited and acquired defects in the immune system lead to immunodeficiencies that make individuals more susceptible to certain infections. • Damage due to immune reactions may reflect attempts to eliminate micro-organisms or self antigen-directed (autoimmune) reactions.