Cells and organs of immune systems

Dr. A K K

Introduction to 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, fungi, and parasites. It also plays a crucial role in identifying and eliminating abnormal cells that may lead to cancer. This system is broadly categorized into the innate and adaptive immune systems, which work together to maintain the body’s defense mechanisms.

Cells of the Immune System

1. Hematopoietic Stem Cells (HSCs)

All immune cells originate from hematopoietic stem cells found in the bone marrow. These multipotent cells differentiate into two main lineages:

  • Myeloid lineage: Gives rise to granulocytes, monocytes, erythrocytes, and platelets.
  • Lymphoid lineage: Produces lymphocytes, including B cells, T cells, and natural killer (NK) cells.

2. Innate Immune Cells

a. Neutrophils
    • Description: The most abundant white blood cells, accounting for 50-70% of all circulating leukocytes.
    • Function: Act as the first responders to microbial invasion. They perform phagocytosis, releasing reactive oxygen species (ROS) to kill pathogens and secrete cytokines to recruit other immune cells. Neutrophils are short-lived and form pus at infection sites.
b. Eosinophils
    • Description: Granulocytes are involved in the defense against parasitic infections and allergic reactions.
    • Function: Release toxic granules and inflammatory mediators like histamines and cytokines to combat multicellular parasites. They are particularly effective against helminths and contribute to chronic inflammation in allergies.
c. Basophils
    • Description: The least abundant granulocytes.
    • Function: Involved in allergic and inflammatory responses by releasing histamine, leukotrienes, and cytokines. Basophils play a critical role in the development of anaphylactic reactions.
d. Monocytes
    • Description: Large, mononuclear leukocytes that circulate in the blood and differentiate into macrophages or dendritic cells upon entering tissues.
    • Function: Perform phagocytosis, antigen presentation, and cytokine production. Monocytes are precursors to tissue-resident macrophages and dendritic cells, contributing to innate and adaptive immunity.
e. Macrophages
    • Description: Derived from monocytes and found in almost all tissues.
    • Function: Engulf pathogens, dead cells, and debris. They also present antigens to T cells and secrete cytokines to modulate immune responses. Macrophages exhibit specialized forms such as Kupffer cells in the liver and alveolar macrophages in the lungs.
f. Dendritic Cells
    • Description: Specialized antigen-presenting cells (APCs).
    • Function: Capture antigens, migrate to lymph nodes, and present them to T cells to initiate adaptive immune responses. Dendritic cells bridge the innate and adaptive immune systems by activating naive T cells.
g. Natural Killer (NK) Cells
    • Description: Lymphocytes of the innate immune system.
    • Function: Destroy virally infected and tumor cells by recognizing stress signals or missing self-markers. NK cells release perforin and granzymes to induce apoptosis in target cells.

3. Adaptive Immune Cells

a. B Lymphocytes (B Cells)
    • Description: Develop in the bone marrow and are responsible for humoral immunity.
    • Function: Produce antibodies that neutralize pathogens, facilitate phagocytosis, and activate the complement system.
      • Plasma cells: Differentiated B cells that secrete large quantities of antibodies.
      • Memory B cells: Long-lived cells that provide immunity against previously encountered pathogens. They rapidly produce antibodies upon re-exposure to specific antigens.
b. T Lymphocytes (T Cells)
    • Description: Develop in the thymus and mediate cellular immunity.
    • Function: Recognize antigens presented by major histocompatibility complex (MHC) molecules.
      • Helper T cells (CD4+ T cells): Coordinate immune responses by releasing cytokines. They assist in activating B cells, cytotoxic T cells, and macrophages.
      • Cytotoxic T cells (CD8+ T cells): Kill infected or cancerous cells by releasing perforin and granzymes, which trigger apoptosis.
      • Regulatory T cells (Tregs): Suppress immune responses to maintain tolerance and prevent autoimmunity. Tregs play a key role in resolving inflammation.
c. Gamma Delta (γδ) T Cells
    • Description: A subset of T cells with a distinct T-cell receptor (TCR).
    • Function: Bridging innate and adaptive immunity by recognizing non-peptide antigens. These cells are found predominantly in mucosal tissues.
d. Natural Killer T (NKT) Cells
    • Description: Hybrid lymphocytes share features of both T cells and NK cells.
    • Function: Recognize lipid antigens presented by CD1d molecules. They contribute to immune regulation, tumor immunity, and host defense against microbial infections.

Organs of the Immune System

1. Primary Lymphoid Organs

a. Bone Marrow
    • Function: Site of hematopoiesis and B cell maturation. It provides a microenvironment rich in growth factors and cytokines that regulate stem cell differentiation.
b. Thymus
    • Function: Site of T cell maturation and selection. Immature T cells undergo positive selection to recognize MHC molecules and negative selection to eliminate self-reactive T cells, ensuring central tolerance.

2. Secondary Lymphoid Organs

a. Lymph Nodes
    • Description: Small, bean-shaped structures distributed along lymphatic vessels.
    • Function: Filter lymph and provide a site for antigen presentation and lymphocyte activation. Lymph nodes are compartmentalized into the cortex, paracortex, and medulla, where distinct immune processes occur.
b. Spleen
    • Description: The largest lymphoid organ is located in the upper left abdomen.
    • Function: Filters blood, removes old erythrocytes, and facilitates immune responses to blood-borne pathogens. The spleen is divided into white pulp (immune function) and red pulp (blood filtration).
c. Mucosa-Associated Lymphoid Tissue (MALT)
    • Description: Lymphoid tissues are found in mucosal surfaces such as the gastrointestinal tract (e.g., Peyer’s patches), respiratory tract, and genitourinary tract.
    • Function: Defends against pathogens at mucosal surfaces. MALT contains specialized cells, including M cells, which transport antigens across the epithelium.
d. Tonsils and Adenoids
    • Description: Lymphoid tissues located in the pharynx.
    • Function: Protect against ingested and inhaled pathogens. These tissues trap antigens and facilitate the activation of local immune responses.
e. Appendix
    • Function: Contains lymphoid tissue that contributes to gut-associated immunity. The appendix is considered a reservoir for beneficial gut bacteria.

Immune System Processes

1. Antigen Recognition

  • Process: Immune cells identify specific pathogen-associated molecular patterns (PAMPs) using pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs). This process initiates downstream signaling pathways that activate immune responses.

2. Phagocytosis

  • Process: Neutrophils and macrophages engulf and digest pathogens. Steps include recognition, ingestion, and degradation within phagolysosomes. Opsonins, such as antibodies and complement proteins, enhance phagocytosis.

3. Antigen Presentation

  • Process: Dendritic cells, macrophages, and B cells process antigens and present them on MHC molecules to T cells. This interaction is critical for initiating adaptive immune responses.

4. Clonal Expansion

  • Process: B and T cells proliferate upon activation and differentiate into effector and memory cells. This ensures a rapid and robust response to pathogens.

5. Cytokine Secretion

  • Description: Cytokines are signaling molecules that regulate immune cell activity, differentiation, and communication.
    • Examples: Interleukins (ILs), interferons (IFNs), and tumor necrosis factors (TNFs). They mediate inflammation, immune cell recruitment, and antiviral responses.

6. Complement Activation

  • Description: A cascade of plasma proteins enhances pathogen clearance by opsonization, inflammation, and membrane attack complex formation. Complement activation occurs via classical, alternative, or lectin pathways.

7. Immunological Memory

  • Description: Adaptive immunity generates memory cells that provide a faster and more robust response upon re-exposure to the same pathogen. This principle underpins the effectiveness of vaccines.

Immune System Dysfunctions

1. Autoimmune Diseases

  • Description: Occurs when the immune system attacks self-tissues.
    • Examples: Rheumatoid arthritis, lupus, type 1 diabetes. Autoimmunity arises due to the breakdown of self-tolerance.

2. Allergies

  • Description: Hypersensitivity reactions to harmless antigens.
    • Examples: Asthma, hay fever, anaphylaxis. Allergies involve IgE-mediated mast cell degranulation and histamine release.

3. Immunodeficiency

  • Description: Occurs when one or more immune system components are defective or absent.
    • Primary: Genetic disorders (e.g., severe combined immunodeficiency).
    • Secondary: Acquired conditions (e.g., HIV/AIDS, chemotherapy-induced). Immunodeficiencies increase susceptibility to infections.

4. Chronic Inflammation

  • Description: Prolonged inflammation can damage tissue and diseases such as atherosclerosis and cancer. Chronic inflammation is often associated with persistent infections or autoimmune disorders.

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