The immune system is the network of biological structures and processes that prevents or limits infection, detects cellular damage, and responds to certain abnormal cells. Its components include physical barriers, specialized immune cells, organs, and soluble molecules. Protection requires both effective responses to threats and mechanisms that prevent unnecessary injury to healthy tissues. In humans, immunity is commonly divided into innate and adaptive defenses, which operate as interacting parts of one system rather than independent layers. (pubweb-prod.niaid.nih.gov)
Organization and immune organs
Most circulating immune cells originate from blood-forming stem cells in the bone marrow. These develop into several types of white blood cells, including granulocytes, monocytes, and lymphocytes. Some circulate through blood and tissues, while others reside in particular organs. Their distribution enables local surveillance and coordinated responses across the body. (ncbi.nlm.nih.gov)
Bone marrow and the thymus are primary lymphoid organs: B cells develop in bone marrow, while T cells mature in the thymus. The lymphatic system carries fluid and immune cells between tissues and the bloodstream. Lymph nodes provide meeting places where immune cells encounter material collected from nearby tissues. The spleen surveys blood-borne material, while mucosa-associated lymphoid tissues monitor surfaces such as the intestine and respiratory tract. These secondary lymphoid tissues support the initiation of adaptive responses. (pubweb-prod.niaid.nih.gov)
Innate defenses
Innate immunity provides rapid protection through barriers, cellular responses, and soluble factors. Skin and mucous membranes restrict entry by pathogens, while antimicrobial substances contribute to defense at exposed surfaces. Innate recognition uses receptors that detect shared microbial features or signals associated with tissue damage. It therefore has defined molecular specificity, although it does not use the highly diversified antigen receptors characteristic of adaptive immunity. (pubweb-prod.niaid.nih.gov)
Neutrophils and macrophages engulf and destroy microbes through phagocytosis. Dendritic cells collect material in tissues and help initiate adaptive responses. Natural killer cells can eliminate certain infected or abnormal cells without first undergoing antigen-specific clonal expansion. Different innate cell populations consequently contribute to immediate defense, communication, and removal of damaged material. (ncbi.nlm.nih.gov)
Inflammation recruits defensive cells and molecules to affected tissues. Signaling proteins called cytokines, including chemokines that guide cell movement, coordinate these activities. The complement system is a network of proteins whose activation can coat microbes for uptake, promote inflammation, and damage susceptible microbial membranes. These mechanisms also cooperate with adaptive defenses. (ncbi.nlm.nih.gov)
Adaptive immunity
Adaptive immunity depends principally on B cells and T cells. Their receptors recognize particular antigens—molecular structures that can be bound by antibodies or recognized by lymphocyte receptors. Receptor diversity arises during lymphocyte development through rearrangement of receptor-encoding gene segments and additional variation at their joining sites. This produces a large repertoire before exposure to a particular infection. (ncbi.nlm.nih.gov)
When appropriate antigen recognition and activation signals occur, responsive lymphocytes multiply through clonal expansion and differentiate into effector cells. Activated B cells can become plasma cells that secrete antibodies. Antibodies bind specific targets and can neutralize toxins or pathogens, facilitate uptake by phagocytes, or activate complement. Antibody-mediated protection is often called humoral immunity. (ncbi.nlm.nih.gov)
T cells provide cell-mediated functions. Helper T cells coordinate other immune cells, including B cells and macrophages, while cytotoxic T cells kill infected cells and certain tumor cells. Most conventional T cells recognize antigen fragments displayed by major histocompatibility complex molecules on cell surfaces. Dendritic cells connect innate recognition with adaptive activation by presenting antigen and supplying additional signals to T cells. (ncbi.nlm.nih.gov)
Immunological memory
Following an adaptive response, some lymphocytes persist as memory cells. Immunological memory enables a faster or more effective response to subsequent exposure to the same antigen. The persistence and degree of protection depend on the pathogen, antigen, and immune response; memory does not imply that every infection produces lifelong protection. (ncbi.nlm.nih.gov)
A vaccine induces active immunity by exposing the immune system to antigens, or enabling their production, without requiring the recipient to experience the corresponding natural disease. Passive immunity instead results from receiving antibodies produced elsewhere, as occurs when maternal antibodies cross the placenta. It can provide immediate protection but is temporary because transferred antibodies are gradually lost. (cdc.gov)
Tolerance and regulation
Immune tolerance limits responses against the body’s own molecules. During lymphocyte development, strongly self-reactive cells may be eliminated, inactivated, or, in the case of developing B cells, undergo receptor editing. These processes help shape a functional repertoire without removing every potentially self-reactive cell. (ncbi.nlm.nih.gov)
Peripheral immune tolerance controls self-reactive cells outside primary lymphoid organs. Mechanisms include functional inactivation and suppression by regulatory T cells. Regulation also restrains excessive responses to genuine threats. When these controls fail, autoimmunity can arise, and immune responses may damage the body’s own tissues. (ncbi.nlm.nih.gov)
Immune dysfunction
Immunodeficiency occurs when immune defenses are impaired. It may result from inherited disorders or acquired causes, including infections such as HIV and treatments that suppress immune activity. Its consequences depend on which components are affected. Allergy involves inappropriate immune responses to substances that are ordinarily harmless, whereas autoimmune disease involves responses directed against self tissues. Immune recognition also contributes to rejection after organ transplantation, when transplanted tissues are recognized as foreign. These disorders illustrate distinct failures of defense, recognition, or regulation rather than a single measure of immune “strength.” (pubweb-prod.niaid.nih.gov)