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Immune System

Discover the amazing ways our immune system protects us from harm, fights off infections, and keeps us healthy throughout our lives.

USMLE Guide: Immune System

This guide provides an informative overview of the immune system for the USMLE exam. It covers essential concepts, components, and functions of the immune system.


The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful pathogens, such as bacteria, viruses, parasites, and fungi. A thorough understanding of the immune system is crucial for medical professionals to diagnose and treat various diseases. This guide will focus on essential concepts and topics related to the immune system.

Components of the Immune System

1. Innate Immunity

  • Physical Barriers: Skin, mucous membranes, and cilia in the respiratory tract.
  • Cellular Components: Neutrophils, macrophages, natural killer (NK) cells, and dendritic cells.
  • Chemical Components: Complement system, interferons, and antimicrobial peptides.

2. Adaptive Immunity

  • Humoral Immunity: Mediated by B cells and antibodies.
  • Cell-Mediated Immunity: Mediated by T cells.

3. Lymphoid Organs

  • Primary Lymphoid Organs: Bone marrow and thymus.
  • Secondary Lymphoid Organs: Spleen, lymph nodes, tonsils, and mucosa-associated lymphoid tissue (MALT).

Immunoglobulins (Antibodies)

1. IgG

  • Structure: Y-shaped monomer with two antigen-binding sites.
  • Function: Most abundant antibody in the blood; provides long-term immunity, crosses the placenta, neutralizes toxins, and opsonizes pathogens.

2. IgM

  • Structure: Pentamer with ten antigen-binding sites.
  • Function: First antibody produced in response to an infection; activates the complement system.

3. IgA

  • Structure: Monomer or dimer with four antigen-binding sites.
  • Function: Found in secretions (saliva, tears, breast milk) and mucous membranes; provides localized immunity.

4. IgE

  • Structure: Monomer with two antigen-binding sites.
  • Function: Involved in allergic reactions and defense against parasites.

5. IgD

  • Structure: Monomer with two antigen-binding sites.
  • Function: Present on the surface of B cells; involved in B cell activation.

Major Histocompatibility Complex (MHC)

1. MHC Class I

  • Structure: Found on all nucleated cells.
  • Function: Presents endogenous antigens (viral or intracellular) to CD8+ T cells; involved in cytotoxic T cell response.

2. MHC Class II

  • Structure: Found on antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells.
  • Function: Presents exogenous antigens (bacterial or extracellular) to CD4+ T cells; involved in helper T cell response.

Hypersensitivity Reactions

1. Type I - Immediate Hypersensitivity

  • Characteristics: IgE-mediated, mast cell activation, and release of histamine.
  • Examples: Allergic rhinitis, asthma, anaphylaxis.

2. Type II - Cytotoxic Hypersensitivity

  • Characteristics: Antibody-mediated, complement activation, and cell destruction.
  • Examples: Hemolytic anemia, autoimmune diseases.

3. Type III - Immune Complex-Mediated Hypersensitivity

  • Characteristics: Immune complexes deposit in tissues, complement activation, and local inflammation.
  • Examples: Systemic lupus erythematosus, glomerulonephritis.

4. Type IV - Delayed Hypersensitivity

  • Characteristics: Cell-mediated response by sensitized T cells.
  • Examples: Tuberculin skin test, contact dermatitis.


Understanding the immune system's components, functions, and associated reactions is crucial for medical professionals to diagnose and manage various diseases. This USMLE guide provides a concise overview that will assist you in comprehending key concepts and scoring well on the exam. Remember to review additional resources and practice questions to solidify your knowledge. Good luck!

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