Immune System Functions, Cells, and Defense Mechanisms
Immunology and the Immune System
Immunology studies the processes involved in recognition and rejection of foreign substances or potentially harmful organisms. The immune system is the complex set of defense mechanisms that allow living things to protect themselves.
Antigen Definition
Antigen: a molecule, often located outside a cell or organism, which is recognized as foreign and that induces an immune response.
The Immune System Components
The immune system comprises leukocytes and antibodies.
The Body's Defenses
- Structural: skin and mucous membranes
- Mechanical: cilia of the respiratory tract, urinary and digestive movements
- Biochemical: HCl in gastric juice; lysozyme in tears, saliva, and urine
- Ecological: commensal microorganisms
Unspecific (Innate) Defense
Inflammation
The inflammatory process consists of three phases:
- Stimulus
- The injured cells release substances that mediate inflammation.
- The mediators act on the capillaries causing:
- increased leukocyte recruitment
- vasodilation
- increased capillary permeability
- activation of phagocytes and chemotaxis
Phagocytes
Phagocytes are leukocytes who are responsible for removing microorganisms and foreign substances. There are two types:
- Granulocytes
- Monocytes — when they leave the blood and enter tissues they are called macrophages
Phagocytic process steps:
- Attachment to the item to be phagocytosed
- Ingestion by pseudopodia
- Death and intracellular digestion
- Removal of undigested remains
The collection of phagocytes and the remains of dead organisms is pus.
Complement System
The complement is a set of about 30 proteins whose function is to complement and enhance the action of antibodies. It performs three functions:
- mediates inflammation
- facilitates phagocytosis and the action of antibodies
- causes breakdown of the plasma membrane of invading cells
Interferon
Interferon helps prevent the spread of infection, acting in two ways:
- prevents virus replication
- activates certain white blood cells (lymphocytes) that can recognize and destroy infected cells
Interferon is species-specific and is not effective against every virus.
Specific (Adaptive) Defenses
Lymphocytes
Lymphocytes are not phagocytic; they produce a specific response against antigens. There are three types:
- B lymphocytes: synthesize antibodies
- T lymphocytes: cause the death of certain altered cells
- Non-B, non-T lymphocytes: destroy target cells non-specifically
Organs of the Immune System
Primary lymphoid organs:
- Thymus — producing T cells
- Bone marrow — producing B cells
Secondary lymphoid organs: lymph nodes, spleen, tonsils, appendix, Peyer’s patches
Mechanism of Action
- Identification and recognition of the foreign antigen
- Activation of lymphocytes — after recognition, they begin to divide actively
- Triggering the immune response