The Immune System: Understanding Its Function and Response

Posted on Nov 13, 2024 in Biology

The Immune System

Overview

The immune system comprises all organs where transformed lymphocytes originate and accumulate. Lymphocytes differentiate from hematopoietic stem cells in bone marrow. Their maturation location determines their type. These maturation sites are called primary lymphoid organs. The thymus produces T cells, while bone marrow produces B lymphocytes. Upon leaving these organs, cells circulate in blood and lymph to secondary lymphoid organs where they accumulate.

Defensive Barriers and Immune Response

The immune system presents a series of barriers that pathogens must overcome to cause infections. These barriers can be categorized into three types:

External Barriers

These are non-specific defenses, acting regardless of the invading agent. They are also called primary barriers as they initially inhibit invasion. They include:

  • Physical Barriers: such as skin, whose thickness and structure hinder pathogen entry; cilia in the respiratory tract; mucus trapping foreign substances in natural openings of the digestive, respiratory, and reproductive systems; and the dragging effect of tears, saliva, and urination.
  • Chemical Barriers: such as the acidic pH in the stomach and small intestine, urogenital tract fluids and gland secretions, and enzymes in tears and saliva that break down bacterial walls.
  • Biological Barriers: consisting of the autochthonous bacterial flora.

Non-Specific Internal Barriers

Also called secondary barriers, these comprise a set of blood cells with phagocytic capacity and biomolecule inactivators, including:

  • Macrophages: can act as antigen-presenting cells.
  • Granulocytes: release histamine and other substances that trigger inflammation.
  • Killer Cells: a type of T cell that destroys cells damaged by viruses or bacteria.
  • Cytokines: proteins secreted by leukocytes that react indiscriminately to any foreign element in the body.

Symptoms of inflammation: redness, increased warmth, swelling, pain.

Cytological effects: leukocyte exit from blood capillaries, migration, release of lytic products by granulocytes.

Internal and external barriers are collectively called natural or innate immunity. This type of immunity lacks memory and always responds the same way.

Specific Internal Barriers

These are specific because the type of protection depends on the invading agent. Lymphocytes recognize specific pathogens or antigens. This barrier constitutes specific immunity, which has two types:

  • Cellular Immunity: defense is conducted by T lymphocytes.
  • Humoral Immunity: based on antibodies produced by B lymphocytes.

This type of immunity has memory, becoming more intense and effective with each subsequent infection attempt. Immune mechanisms cooperate during an immune response, but we can study them separately.

Antigens

Characteristics

Antigens are substances with the following characteristics:

  • They are exogenous.
  • They are immunogenic.
  • They react specifically with antibodies.
  • They have varied chemical nature and high molecular weight.
  • They are located on the surface of a pathogen or in substances it produces and releases.

Structure

Antigens have two types of structures:

  1. Antigen carrier, which is a macroprotein.
  2. Antigenic determinants, which are small molecules linked to the carrier, with a specific spatial pattern recognizable by an antibody. These epitopes are responsible for the antigen’s specificity.

Antigens are versatile because a single antigenic molecule can produce different antibody molecules, as many as it has different antigenic determinants.

Types of Antigens

  • Autoantigen: from the individual themselves, native. Example: cancer cells.
  • Alloantigen: from individuals of the same species. Example: red blood cells, heart transplant.
  • Xenoantigen: from a different species than the receiver. Example: viruses, bacteria, mites.

Haptens are molecules capable of binding specifically to certain antibodies but are not considered antigens because they are not immunogenic and do not induce antibody synthesis.

Antibodies

Structure

Antibodies possess three characteristics:

  1. They are glycoproteins called immunoglobulins, produced by B cells.
  2. They are produced in response to a specific antigen.
  3. They are found in all body fluids.

Immunoglobulins are composed of four polypeptide chains: two identical heavy chains and two identical light chains, bound together by disulfide bridges, forming a symmetrical and flexible Y-shaped structure.

Antibody molecules are very similar, but structural differences are responsible for the specific combination with epitopes. Two regions can be distinguished:

  • Variable Region: has a three-dimensional folding that allows it to fit specifically with a particular epitope.
  • Constant Region: activates phagocytes and the complement system.

Role of Antibodies

Antibodies function by forming antigen-antibody complexes. They:

  • Inactivate the antigen and neutralize its toxicity.
  • Cause precipitation.
  • Activate the complement system.
  • Attract macrophages and increase the activity of Tc and NK cells.

Antigen-Antibody Reaction

The formation of the antigen-antibody complex is a highly specific reaction aimed at eliminating pathogen activity. Antibodies recognize pathogens by binding to antigens on their surface or to secreted toxins. Antigen-antibody complexes can activate the complement system, whose proteins bind to and inactivate the pathogen by completely covering it. This phenomenon is called opsonization.

Specific Immune Response

The specific immune response has two forms:

Humoral Response

Also known as antibody-mediated immunity, it involves the synthesis of antibodies by B cells, which are released into the extracellular medium. When an antigen enters the body, it eventually encounters a B lymphocyte with an antibody that can react with it. The binding of the lymphocyte to the antigen through its surface antibodies stimulates the B cell, causing it to divide and differentiate into two types of lymphocyte cells:

  • Plasma Cells: mature B lymphocytes, larger than immature ones and with a relatively short lifespan. They develop an extensive RER that synthesizes and exports large quantities of antibodies. These cells do not leave the lymph nodes.
  • Memory Cells: immature B cells that remain in circulation and continue producing small amounts of antibodies long after the infection.

Cell Response

Also known as cell-mediated immunity, it is based on the activity of lymphocytes and macrophages. The mechanism is as follows:

  1. When an antigen invades the body, macrophages engulf and digest it.
  2. Macrophages synthesize a protein complex called MHC (Major Histocompatibility Complex).
  3. MHC molecules bind peptides and transport them to the macrophage surface, where they remain until recognized by a T cell.
  4. T lymphocytes have specialized receptors on their plasma membranes that recognize foreign peptide fragments attached to MHC molecules on the surface of other cells.

Different T lymphocytes are distinguished by the proteins on their membranes and their mode of action.

Types of T Lymphocytes

  • CD8 Tc Cells: have CD8 on their surface and release proteins that directly or indirectly destroy and lyse the infected cell.
  • CD4 T Lymphocytes: recognize peptides presented on the surface of macrophages or other cells that have captured antigens.
  • Memory T Cells: activated T lymphocytes that remain in lymphoid tissue and continue dividing, providing memory for years. If the pathogen re-infects the body, these cells proliferate rapidly and destroy it before it can establish itself and cause disease.

Immunization

Immunization is the state of an individual’s resistance against infections caused by the invasion of foreign macromolecules and pathogens.

  • Natural Passive Immunization: acquired during embryonic development and lactation through maternal antibodies.
  • Natural Active Immunization: acquired after recovering from an infectious disease.
  • Artificial Passive Immunization: acquired by administering sera.
  • Artificial Active Immunization: acquired by administering vaccines.

Vaccination

Vaccination is the artificial inoculation of a preparation containing the pathogen or toxin, which has lost its pathogenic nature but retains its antigenicity, causing the production of antibodies. It is a preventive measure that promotes the formation of memory cells.

Serum Therapy

Serum therapy involves injecting serum containing specific antibodies against a particular disease, formed by another body. It is a treatment or cure but does not provide immunization because no memory cells are formed.

Antibiotics

Antibiotics are substances produced and released by certain microorganisms that prevent the growth of other microbes, either by interfering with their metabolic activity or by attacking their membranes, causing lysis.