Mechanisms of Autoimmunity Autoimmunity arises from a breakdown in the immune system's ability to distinguish self from non-self. Several mechanisms contribute
Mechanisms of Autoimmunity Autoimmunity arises from a breakdown in the immune system's ability to distinguish self from non-self. Several mechanisms contribute to the development of autoimmune diseases. One primary cause is the loss of self-tolerance , where failures in central (thymus/bone marrow) or peripheral tolerance mechanisms allow autoreactive lymphocytes to survive and become active. Another mechanism is molecular mimicry , where pathogens express antigens that closely resemble self-antigens . This similarity can trigger cross-reactive immune responses, leading the immune system to attack the body's own tissues. Epitope spreading describes a process where an initial immune response to a specific antigen broadens over time, leading to responses against other self-antigens. The release of sequestered antigens can also initiate autoimmunity. Normally hidden self-antigens, such as those found in the eye or brain, can become exposed due to tissue damage or infection, subsequently being recognized as foreign by the immune system. Lastly, aberrant expression of MHC molecules , particularly the upregulation of MHC class II on cells that are not typical antigen-presenting cells (non-APCs), can lead to the presentation of self-antigens to T cells, thereby initiating an autoimmune response. Alternative Complement Pathway The alternative complement pathway is an important component of innate immunity, activated independently of antibodies. It begins with initiation through the spontaneous hydrolysis of C3 , producing C3(H2O) , which then binds to factor B . This binding leads to the formation of C3 convertase : factor D cleaves factor B, resulting in the complex C3bBb , which serves as the alternative pathway's C3 convertase. An amplification loop quickly ensues, where C3bBb cleaves more C3, generating additional C3b and thereby amplifying the complement response. Properdin plays a crucial role in stabilizing the C3bBb complex, particularly on microbial surfaces, enhancing its activity. The pathway culminates in the terminal pathway , where C3b joins the C3bBb complex to form C5 convertase (C3bBbC3b) . This C5 convertase then initiates the formation of the membrane attack complex (MAC) , a pore-forming structure that can lyse target cells. T-Cell Receptor (TCR) Characteristics The T-cell receptor (TCR) is a crucial molecule on the surface of T cells, responsible for recognizing specific antigens. Its structure typically consists of two polypeptide chains, either α and β chains or, less commonly, γ and δ chains . Each chain possesses both variable (V) and constant (C) regions , contributing to its antigen-binding specificity and structural integrity. For antigen recognition , the TCR does not bind directly to free antigens. Instead, it specifically binds to peptide-MHC complexes presented on the surface of antigen-presenting cells (APCs) . Each TCR is monovalent , meaning it recognizes a single, specific antigenic peptide presented in the context of an MHC molecule. A key distinction from antibodies is that TCRs have no soluble form ; they are exclusively membrane-bound and are not secreted by T cells. Upon antigen binding, the TCR itself does not directly transmit signals into the T cell. Instead, it requires the CD3 complex to transduce these signals, initiating the intracellular cascade necessary for T cell activation and function. Classification of Immunodeficiencies Immunodeficiencies are conditions where the immune system's ability to fight infectious diseases is compromised. They can be broadly classified based on the primary component of the immune system affected: B cell (Humoral) Deficiencies involve impaired antibody production, leading to increased susceptibility to bacterial infections. An example is X-linked agammaglobulinemia (Bruton’s disease) . T cell (Cellular) Deficiencies compromise cell-mediated immunity, often resulting in susceptibility to viral, fungal, and intracellular bacterial infections. DiGeorge syndrome , characterized by thymic hypoplasia, is a notable example. Combined Immunodeficiencies involve defects in both B and T cell arms of the adaptive immune system, leading to severe and life-threatening infections. Severe Combined Immunodeficiency (SCID) is a classic example. Phagocytic Defects compromise the function of phagocytic cells like neutrophils and macrophages, making individuals vulnerable to bacterial and fungal infections. Chronic Granulomatous Disease (CGD) is an example. Complement Deficiencies involve defects in components of the complement system, which can impair host defense against certain pathogens and immune complex clearance. A C3 deficiency , for instance, can lead to recurrent bacterial infections. Tissue-Specific Macrophages Macrophages are versatile immune cells that reside in various tissues, where they differentiate and adopt specialized functions. These tissue-specific macrophages are crucial for local immune surveillance, pathogen clearance, and tissue homeostasis. Examples include: Kupffer cells , found in the liver , which are vital for clearing pathogens and debris from the portal circulation. Alveolar macrophages , located in the lungs , responsible for engulfing inhaled particles and microorganisms. Microglia , the resident macrophages of the central nervous system , playing roles in brain development, maintenance, and response to injury or infection. Langerhans cells , found in the skin and mucosa , which function as antigen-presenting cells. Splenic macrophages , particularly in the red pulp of the spleen , involved in filtering blood and removing old or damaged red blood cells and pathogens. Osteoclasts , specialized macrophages in bone , responsible for bone resorption and remodeling. Antiviral Immune Response The body mounts a coordinated antiviral immune response involving both innate and adaptive immunity to combat viral infections. Innate immunity provides the first line of defense. This includes the rapid production of interferons (IFN-α/β) , which act to inhibit viral replication in infected and neighboring cells. Natural Killer (NK) cells are activated to directly kill virally infected cells. Additionally, dendritic cells play a crucial role by capturing and presenting viral antigens, bridging the gap to the adaptive immune response. Adaptive immunity provides a more specific and long-lasting defense. Cytotoxic T lymphocytes (CD8+) are essential for killing virally infected cells that present viral peptides on their surface. Helper T cells (CD4+) support the immune response by activating B cells, which then differentiate into plasma cells to produce neutralizing antibodies . These antibodies can block viral entry into host cells and facilitate viral clearance. Hypersensitivity Reactions Hypersensitivity reactions are exaggerated or inappropriate immune responses that cause tissue damage. They are classified into four main types: Type I (Immediate) Hypersensitivity is IgE mediated . It involves the rapid degranulation of mast cells upon re-exposure to an allergen, leading to the release of inflammatory mediators. Examples include anaphylaxis and asthma . Type II (Cytotoxic) Hypersensitivity is mediated by IgG or IgM antibodies that bind to antigens on cell surfaces. This binding can lead to cell destruction through complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC) . An example is autoimmune hemolytic anemia . Type III (Immune Complex) Hypersensitivity occurs when antigen-antibody complexes form in the circulation and deposit in various tissues, such as blood vessel walls, glomeruli, or joints. These deposits trigger inflammation and tissue damage. Systemic lupus erythematosus (SLE) is a classic example. Type IV (Delayed-Type) Hypersensitivity is unique as it is T lymphocyte mediated , primarily by Th1 cells , rather than antibodies. The reaction typically develops 24–72 hours after antigen exposure. Examples include the tuberculin test and contact derma