The Immune System

The system of defence mechanisms that protects the body against harmful agents is called immune system.

The hereditary, non-specific protection present from birth is called innate (natural) immunity.

The immunity developed during an individual’s lifetime after exposure to antigens is called acquired (specific) immunity.

The antibody-mediated defence mechanism is called humoral immune response.

The defence mechanism involving lymphocytes and macrophages without antibody production is called cell-mediated (cellular) immune response.

• The immune system defends the body against infectious agents and consists of lymphoid organs, tissues, cells, and soluble molecules like antibodies.
• Primary lymphoid organs (bone marrow and thymus) are the sites where immature lymphocytes originate, develop, and differentiate into antigen-sensitive cells.
• Secondary lymphoid organs (spleen, lymph nodes, tonsils, and MALT) provide the specific sites where mature lymphocytes interact with antigens and proliferate.
• The spleen acts as a blood filter, trapping blood-borne microorganisms, while lymph nodes trap antigens in the lymphatic fluid, triggering localised immune responses.
• The humoral immune response is driven by specific lymphocytes that produce circulating antibodies to bind, neutralise, and eliminate foreign antigens.
• The cellular immune response does not involve circulating antibodies, but instead relies on the direct cooperation of lymphocytes and macrophages to destroy pathogenic organisms.