immune

The immune system is the body's defense against pathogens and abnormal cells. It comprises cells, tissues, and organs that detect and respond to foreign invaders while maintaining tolerance to self.

Immunity is broadly divided into innate and adaptive components. Innate immunity provides immediate, non-specific defense: skin and mucous barriers, phagocytic cells (neutrophils, macrophages), dendritic cells, natural killer cells, and the complement system. It recognizes common features of pathogens through pattern recognition receptors. Adaptive immunity is specific and tailored, involving lymphocytes: B cells produce antibodies that neutralize pathogens; T cells regulate responses or destroy infected cells. Antibodies can mark pathogens for attack or neutralize toxins.

Lymphoid organs enable immune responses: bone marrow produces blood cells including B cells; thymus educates T cells; spleen and lymph nodes coordinate responses. Exposure to antigens leads to memory: upon re-exposure, faster and stronger responses. Vaccines simulate infection to induce adaptive immunity without disease. Passive immunity transfers immunity passively, e.g., maternal antibodies or therapeutic antibodies.

Immune dysregulation can lead to immunodeficiency, autoimmunity, or hypersensitivity (allergies). Factors like age, nutrition, genetics, and infections (for example HIV) influence immune function. The topic encompasses research across immunology, vaccine science, and clinical medicine.