The Immune System

Overview: Reconnaissance, Recognition, and Response

The immune system protects animals from a wide variety of pathogens through two major types of defense: innate immunity and acquired (adaptive) immunity. These mechanisms work together to recognize and respond to threats, ensuring survival and health.

• Innate immunity: Present from birth, provides nonspecific defense against pathogens.

• Acquired immunity: Develops after exposure to specific pathogens, providing targeted responses.

Innate Immunity: Broad Defenses Against Infection

Innate immunity is the first line of defense, acting rapidly and nonspecifically against pathogens. It includes external barriers and internal cellular and chemical defenses.

• External barriers: Skin, mucous membranes, and secretions prevent pathogen entry.

• Internal defenses: Macrophages, neutrophils, natural killer cells, antimicrobial proteins, and inflammatory responses.

External Defenses

Physical and chemical barriers are crucial in preventing infection.

• Skin: Acts as a tough, impermeable barrier.

• Mucous membranes: Produce mucus to trap microbes; ciliated cells sweep mucus and trapped particles away from lungs.

• Secretions: Acidic pH (3-5) and proteins like lysozyme inhibit microbial growth.

Internal Cellular and Chemical Defenses

Once pathogens breach external barriers, internal defenses are activated.

• Phagocytic cells: Engulf and destroy pathogens via phagocytosis.

• Natural killer cells: Attack virus-infected and cancer cells, triggering apoptosis.

• Antimicrobial proteins: Complement system and interferons attack microbes and inhibit viral reproduction.

Phagocytosis Mechanism

Phagocytes attach to pathogens, engulf them, and destroy them within lysosomes.

• Surface receptors recognize pathogens.

• Engulfment forms a vacuole that fuses with a lysosome.

• Enzymes and toxic molecules (e.g., nitric oxide, lysozyme) degrade pathogens.

The Lymphatic System

The lymphatic system is a network of vessels and organs that house immune cells and facilitate pathogen defense.

• Lymph nodes: Masses of lymphocytes and macrophages filter lymph.

• Organs: Spleen, tonsils, adenoids, Peyer's patches, and appendix play roles in immunity.

Inflammatory Response

Inflammation is a local response to injury or infection, characterized by redness, heat, swelling, and pain.

• Histamine and other chemicals: Released from injured cells, increase blood flow and vessel permeability.

• Phagocytes and antimicrobial proteins: Enter tissues to combat pathogens.

• Fever: Systemic response that can enhance defense mechanisms.

Innate Immunity in Invertebrates

Invertebrates use similar mechanisms, such as physical barriers (exoskeleton) and hemocytes in hemolymph, but generally lack acquired immunity.

Acquired Immunity: Specific Defenses Against Infection

Acquired immunity is mediated by lymphocytes and provides highly specific responses to pathogens.

• Antigen: Any foreign molecule recognized by lymphocytes.

• Epitope: The specific region of an antigen recognized by a lymphocyte or antibody.

Antigen Recognition by Lymphocytes

B cells and T cells are the two main types of lymphocytes, each with unique antigen receptors.

• B cell receptors: Bind to intact antigens; secreted antibodies are similar in structure.

• T cell receptors: Bind to antigen fragments presented by MHC molecules on cell surfaces.

The Role of MHC Molecules

MHC molecules present antigen fragments to T cells, enabling recognition and response.

• Class I MHC: Found on most nucleated cells; present antigens to cytotoxic T cells.

• Class II MHC: Found on antigen-presenting cells; present antigens to helper T cells.

Lymphocyte Development and Diversity

Lymphocytes originate from stem cells in bone marrow and differentiate into B or T cells based on their maturation site.

• B cells: Mature in bone marrow.

• T cells: Mature in thymus.

• Diversity: Generated by random gene rearrangement, producing millions of unique antigen receptors.

Clonal Selection and Memory

Upon antigen exposure, lymphocytes undergo clonal selection, producing effector and memory cells for rapid future responses.

• Primary response: Initial activation and proliferation.

• Secondary response: Faster and stronger due to memory cells.

Humoral and Cell-Mediated Immunity

The immune system uses humoral and cell-mediated responses to defend against different threats.

• Humoral response: B cells produce antibodies that neutralize pathogens in extracellular fluids.

• Cell-mediated response: Cytotoxic T cells destroy infected or abnormal cells.

Helper T Cells

Helper T cells activate both humoral and cell-mediated responses by secreting cytokines and interacting with antigen-presenting cells.

Cytotoxic T Cells

Cytotoxic T cells recognize infected or cancerous cells via class I MHC and destroy them by releasing proteins that induce apoptosis.

B Cells and Antibody Production

B cells, upon activation, differentiate into plasma cells that secrete large quantities of antibodies, and memory B cells for long-term immunity.

Antibody Classes

There are five major classes of antibodies (immunoglobulins), each with distinct functions and distributions:

• IgM: First produced, effective in complement activation.

• IgG: Most abundant, crosses placenta for passive immunity.

• IgA: Present in secretions, provides localized defense.

• IgE: Triggers allergic reactions.

• IgD: Acts as antigen receptor on naive B cells.

Antibody-Mediated Disposal of Antigens

Antibodies neutralize pathogens, promote agglutination, precipitate soluble antigens, and activate the complement system for cell lysis.

Active and Passive Immunization

Immunity can be acquired actively (through infection or vaccination) or passively (through transfer of antibodies).

• Active immunity: Long-lasting, develops after exposure or immunization.

• Passive immunity: Immediate, short-term protection via antibody transfer (e.g., maternal antibodies, antivenin).

Self vs Nonself: Tissue Transplantation and Blood Groups

The immune system distinguishes self from nonself, which is critical in tissue transplantation and blood transfusions.

• Transplant rejection: Recipient's immune system attacks transplanted tissues.

• Blood groups: Antigens on red blood cells determine compatibility; mismatched transfusions can be fatal.

• Rh factor: Can cause complications in pregnancy if mother is Rh-negative and fetus is Rh-positive.

Example: Type O blood is the universal donor, while type AB is the universal recipient.

Additional info: The immune system's ability to distinguish self from nonself is essential for preventing autoimmune diseases and ensuring safe medical procedures such as transfusions and transplants.