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Immune System Components And Functions

Discover how the various components of the immune system work together to protect your body, and unlock the secrets behind their vital functions.

USMLE Guide: Immune System Components And Functions


The immune system is a complex network of cells, tissues, and organs that work together to protect our body from harmful pathogens and foreign substances. Understanding the components and functions of the immune system is essential for medical professionals, particularly those preparing for the United States Medical Licensing Examination (USMLE). This guide aims to provide a comprehensive overview of the immune system's components and their functions to aid in your preparation.

Table of Contents

  1. Innate Immunity
    • Physical and Chemical Barriers
    • Cellular Components
    • Inflammatory Response
  2. Adaptive Immunity
    • Humoral Immunity
    • Cell-Mediated Immunity
    • Antigen Recognition
  3. Immunoglobulins
    • IgG
    • IgM
    • IgA
    • IgD
    • IgE
  4. Cells of the Immune System
    • Macrophages
    • Neutrophils
    • Natural Killer (NK) Cells
    • T Lymphocytes (T cells)
    • B Lymphocytes (B cells)
  5. Major Histocompatibility Complex (MHC)
    • MHC Class I
    • MHC Class II
  6. Cytokines
    • Interleukins
    • Interferons
    • Tumor Necrosis Factor (TNF)

1. Innate Immunity

Physical and Chemical Barriers

The skin, mucous membranes, and secretions such as tears and saliva act as physical barriers to pathogens. Additionally, stomach acid, enzymes, and antimicrobial peptides provide chemical barriers to inhibit the growth of microorganisms.

Cellular Components

Innate immunity involves various cellular components, including phagocytes (macrophages and neutrophils) that engulf and destroy pathogens, as well as natural killer (NK) cells that can directly kill infected cells.

Inflammatory Response

Inflammation is a vital component of innate immunity. It is initiated by the release of chemical mediators, leading to increased blood flow, recruitment of immune cells, and activation of the complement system.

2. Adaptive Immunity

Humoral Immunity

Humoral immunity involves the production of antibodies by B cells, which are specific proteins that recognize and neutralize pathogens. Antibodies can be produced in response to infections or through vaccination.

Cell-Mediated Immunity

Cell-mediated immunity relies on the action of T cells, which directly attack infected cells or coordinate immune responses. T cells play a crucial role in fighting intracellular pathogens and in the immune response against cancer cells.

Antigen Recognition

The adaptive immune system recognizes specific antigens through antigen-presenting cells (APCs), which process and present antigens to T cells. This recognition is mediated by the interaction between T cell receptors (TCRs) and major histocompatibility complexes (MHCs).

3. Immunoglobulins


IgG is the most abundant immunoglobulin in the blood and provides long-term protection against infections. It can cross the placenta, conferring passive immunity to the fetus.


IgM is the first antibody produced during an initial immune response. It is responsible for agglutination of antigens and activates the complement system.


IgA is primarily found in secretions such as saliva, tears, and breast milk. It plays a crucial role in mucosal immunity, preventing pathogen colonization.


IgD is present on the surface of B cells and is involved in the activation and differentiation of B cells during the immune response.


IgE is involved in allergic reactions and defense against parasitic infections. It triggers the release of histamine from mast cells and basophils.

4. Cells of the Immune System


Macrophages are phagocytic cells that engulf and destroy pathogens. Additionally, they play a vital role in antigen presentation, activating the adaptive immune response.


Neutrophils are the most abundant white blood cells and are essential in the early stages of infection. They phagocytose pathogens and release antimicrobial substances.

Natural Killer (NK) Cells

NK cells are capable of recognizing and killing infected or cancerous cells directly. They play a crucial role in immune surveillance.

T Lymphocytes (T cells)

T cells coordinate immune responses and directly attack infected cells. They are divided into helper T cells (CD4+) and cytotoxic T cells (CD8+), each with specific functions.

B Lymphocytes (B cells)

B cells produce antibodies and are responsible for humoral immunity. They can differentiate into plasma cells that secrete antibodies or

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