Immune system

The body protects itself from infectious organisms through the host defense system. Three lines of defense include:
• physical and chemical barriers to infection
• the inflammatory response
• the immune response.

Immune system structures
• Composed of lymph nodes, thymus, spleen, and tonsils.

Types of immunity
• Cell-mediated immunity — T cells respond to antigen.
• Humoral immunity — B cells respond to antigen.

Types of immune disorders
• Immunodeficiency disorders
• Hypersensitivity disorders
• Autoimmune disorders

How immune disorders develop
• Allergic rhinitis — type I, IgE-mediated response to allergen.
• Anaphylaxis — type I allergic reaction from systemic exposure to antigen.
• Human immunodeficiency virus disease — ribonucleic acid retrovirus replicates within the CD4+ cell, resulting in cell death.
• Lupus erythematosus — antibodies are produced against body’s own cells; normal immunity suppressed by formed antigen-antibody complex that results in tissue damage.
• Rheumatoid arthritis — exposure to an antigen causes altered IgG antibodies to develop; the body doesn’t recognize antibodies, and an antibody forms against them.