Hematology and Immunology Overview

Immunology Overview

• Immunology studies the components and function of the immune system, essential for defending against foreign pathogens, microbial toxins, and tumor cells.
• The immune system is a complex network of cells, tissues, organs, and soluble factors that provide both non-specific and specific defenses.

Structure of the Immune System

• Immune system organs are distributed throughout the body, facilitating the recognition and destruction of foreign substances.
• The reticulo-endothelial system (RES) allows immune cells to move outside the blood and lymphatic systems.
• Key systems involved include the extracellular fluid system, the blood circulatory system, and the lymphatic system, which houses lymphoid organs.

Lymphoid Organs

• Primary lymphoid organs, such as bone marrow and thymus, are sites for lymphocyte development and maturation.
• Secondary lymphoid organs (spleen, lymph nodes) initiate adaptive immune responses and facilitate lymphocyte proliferation.

Functions of the Immune System

• Defends against invading pathogens, removes damaged tissues, and identifies and destroys abnormal cells, crucial for cancer prevention.

Types of Immunity

• Immunity encompasses innate (non-specific) and acquired immunity (specific).
• Innate immunity provides immediate defense against pathogens through epithelial barriers, phagocytes, and complement proteins.

Innate Immunity

• Acts as the first defense line; components include immediate microbial combat, instructing the adaptive immune system, clearing dead tissue, and initiating repair processes.
• It distinguishes self from non-self without generating immunogenic memory.

Complement System

• A collection of proteins playing a central role in innate immunity, consisting of three activation pathways.
• Functions include opsonization, inflammation (via C3a and C5a), and cell lysis through membrane attack complexes.

Adaptive Immunity

• Develops targeted resistance against specific foreign substances such as bacteria and viruses and involves lymphocytes.
• Composed of cellular immunity (T cells) and humoral immunity (B cells and antibodies).

Lymphocyte Development

• Lymphocytes develop in the bone marrow and are categorized into T and B lymphocytes.
• T lymphocytes (80% of lymphocytes) mature in the thymus; key types include CD4+ (helper) and CD8+ (cytotoxic) cells.
• B lymphocytes (20% of lymphocytes) are responsible for antibody production and are activated in the bone marrow.

Functions of T Cells

• CD8+ T cells are cytotoxic, while CD4+ T cells assist in the activation and proliferation of other immune cells.
• Cellular immunity is vital for organ graft rejection, anti-tumor responses, and collaboration with B cells in humoral immunity.

B Cells and Humoral Immunity

• B cells, constituting 20% of lymphocytes, produce antibodies (immunoglobulins) through plasma cells derived from activated B cells.
• Memory cells enhance response speed and antibody affinity upon re-exposure to antigens.

Antibody Functions

• Antibodies are proteins produced by B cells that enhance phagocytosis (opsonization), block pathogen adhesion (neutralization), activate the complement system, and increase inflammation.

Antibody Structure

• Composed of two heavy chains and two light chains, forming a Y-shaped molecule with variable and constant regions.
• Ig classes include IgD, IgM, IgG, IgE, and IgA, each serving specific functions in immune response.

Immunoglobulin Classes

• IgD: Found on B-cell surfaces; function mainly unknown.
• IgM: First antibody class produced during a primary response; effective in complement activation.
• IgG: Most abundant in serum; provides passive immunity to fetuses and activates complement.
• IgE: Mediates immediate hypersensitivity reactions; binds to mast cells and basophils.
• IgA: Predominant in external secretions (milk, saliva); provides passive immunity to infants.