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The immune system comprises innate and adaptive defenses that protect the body from disease. The innate system includes physical barriers like skin and mucus membranes, plus phagocytic cells such as macrophages that mount inflammatory responses. The adaptive system employs lymphocytes and antibodies to specifically target infectious agents after prior exposure, forming immunological memory for stronger future responses.

Physical barriers form the immune system's first line of defense by preventing pathogen entry. Skin is acidic and contains bactericidal chemicals that inhibit bacterial growth. Mucus-coated hairs in the nasal passages trap inhaled particles, while secretions like saliva and mucus coat the gastrointestinal tract lining, collectively blocking most invaders before they penetrate deeper tissues.

When physical barriers fail, the second line of internal defense activates. Phagocytic cells like macrophages and mast cells, along with other non-specific cells and chemicals, mount an inflammatory response to prevent infection spread. This inflammatory response recruits additional immune cells to the entry site and can activate the adaptive immune system if the threat persists.

The innate immune system acts quickly and non-specifically within minutes to hours using physical barriers and diverse cell types. The adaptive immune system responds over days with high specificity, employing B cells and T cells that target particular pathogens. Crucially, the adaptive system learns and retains memory of previous infections, enabling faster and stronger responses upon re-exposure to the same pathogen.

B cells constitute the humoral branch of adaptive immunity and can directly destroy foreign particles or differentiate into plasma cells that release antibodies targeting invaders. T cells perform cell-mediated immune responses, carrying surface receptors specific to single antigens. After encountering an antigen, T cells can stimulate other immune components or actively destroy infected or cancerous cells.

Immunological memory occurs when B and T cells persist after infection clearance, retaining information about previously encountered pathogens. Upon re-exposure to the same foreign element, these memory cells mount a stronger and faster immune response. This adaptive learning within an organism's lifetime enables vaccines to provide long-term protection against specific diseases.

Autoimmune diseases develop when the immune system fails to distinguish the body's healthy tissue from foreign elements. In Type 1 diabetes mellitus, immune cells produce antibodies that attack insulin-producing beta cells in the pancreas, destroying them and preventing blood sugar regulation. Early diagnosis and management through insulin treatment, diet, and exercise help prevent complete beta cell destruction.