The Human Body Module 12

Overview of the Immune System and Lymphatic System

The body's defenses are structured similarly to a well-built fortress. Key components include blood checks and cleaning via lymph vessels, lymph nodes, and the spleen. The immune system utilizes both innate and acquired defenses against pathogens and possesses the capability to develop immunity to specific diseases. Each cell generates proteins to signify whether it is native or foreign.

Types of Acquired Immunity

Acquired immunity can be categorized into four types:

  1. Active Natural Immunity: Immunity gained through exposure to a pathogen, exemplified by recovering from a disease.

    • Example: Initial exposure to the chickenpox virus results in illness, but subsequent infections are prevented due to the formation of memory B cells and T cells.

  2. Active Artificial Immunity: Immunity developed through vaccinations. Vaccines stimulate the immune system to create memory cells without causing the disease.

  3. Passive Natural Immunity: Occurs when antibodies are transferred from mother to baby during pregnancy or via breastfeeding.

    • Example: IgG antibodies cross the placenta, while IgA antibodies are present in breast milk.

  4. Passive Artificial Immunity: Involves the transfer of antibodies from an individual exposed to a pathogen to another individual. Typically temporary, this method can employ animal-derived antibodies also.

    • Example: Treatment for rabies or tetanus can involve anti-serum from previously exposed or immunized individuals.

Autoimmunity

Autoimmunity refers to the immune system mistakenly attacking its own cells, often due to confusion between the Major Histocompatibility Complex (MHC) of self and non-self cells.

  • Example: In multiple sclerosis, the lymphatic system misidentifies neuroglia as foreign, resulting in damage to the myelin sheath and loss of muscle control.

Acquired Immunity: Cell-Mediated Immunity

Cell-mediated immunity arises primarily through T lymphocytes, which mature in the thymus (with "T" signifying thymus). Unlike B cells, T cells recognize antigens directly on the surface of infected or cancerous cells via their antigen receptors.

Types of T Cells

  • Cytotoxic T Cells: Target and destroy infected or foreign cells through a process that causes lysis.

  • Helper T Cells: Assist in activating B cells and other immune cells by secreting signaling molecules known as cytokines.

  • Delayed Hypersensitivity T Cells: Involved in allergic reactions and inflammation, releasing chemicals to promote response in affected areas.

T Cell Activation

When T cells encounter altered MHC proteins typical in infected or cancerous cells, they generate effector and memory T cells. Effector T cells spearhead attack, while memory T cells ensure long-term immunity, akin to memory B cells.

Acquired Immunity: Humoral Immunity

Humoral immunity involves antibodies dissolved in plasma that are produced by B cells, important for targeting pathogens. Antibodies consist of four polypeptide chains structured in a Y shape, featuring variable regions that specifically bound to unique antigens.

Classes of Antibodies

  1. IgG: Promotes phagocytosis and neutralizes toxins.

  2. IgM: Forms complexes and activates complement proteins.

  3. IgA: Secretes into bodily fluids to protect mucosal surfaces.

  4. IgE: Initiates inflammation through basophil binding.

  5. IgD: Inactivates antigens by binding.

B Cell Role

B cells produce antibodies, proliferating into:

  • Plasma B Cells: Release antibodies into the bloodstream.

  • Memory B Cells: Provide long-term immunity against re-exposure to the same antigen, vital for vaccination efficacy.

Vaccines and Immunity Development

Vaccines elicit immune responses without causing disease, either using weakened pathogens or synthetic analogs. This leads to the generation of memory cells capable of mounting rapid secondary responses.

Importance of Memory B Cells

Memory B cells persist for many years, facilitating quick responses to previously encountered pathogens. Booster vaccinations may be necessary as these cells do not last indefinitely.

Major Histocompatibility Complex (MHC)

MHC proteins serve as cell "fingerprints." They are formed by genetic combinations of glycoproteins and are essential for T cell recognition of self from non-self. Recognition by the lymphatic system prevents attacks on the body's own cells.

Innate Immunity

Innate immunity serves as the immediate first line of defense, employing non-specific mechanisms to combat a broad spectrum of pathogens.

Key Components of Innate Immune Response

  1. Complement System: A series of proteins that interact to destroy pathogens, stimulate inflammation, and enhance phagocytosis. Activation can occur through direct pathogen interaction or through antibodies.

  2. Interferon: Proteins released by infected cells that inhibit viral replication and signal neighboring cells to enhance their defenses.

  3. White Blood Cells: Various types including neutrophils, macrophages, natural killer cells, and eosinophils that respond to pathogens.

    • Neutrophils: First responders that carry out phagocytosis.

    • Macrophages: Larger phagocytic cells that continue the immune response.

    • Eosinophils: Play a role in reducing inflammation.

Inflammation

Inflammation enhances immune response effectiveness by increasing vascularity and recruiting immune cells to affected areas. It can be local (specific to one region) or systemic (throughout the body), the latter often accompanied by fever due to pyrogen release.

Fever Utility

Although uncomfortable, fever accelerates immune system activity and is often detrimental to invading pathogens like bacteria.

Role of the Lymphatic System

The lymphatic system fulfills roles including fluid balance, fat absorption from the digestive system, and immunological defense.

Fluid Balance

The lymphatic system collects excess interstitial fluid from tissues, preventing edema and returning it to the circulatory system via lymph vessels and nodes.

Absorption of Fats

Specialized vessels (lacteals) in the small intestine absorb dietary fats, converting lymph into chyle, which eventually enters the bloodstream.

Filtering through Lymph Nodes

Lymph nodes filter lymph for foreign invaders, facilitating the immune response by producing lymphocytes as necessary. This filtering process enables greater monitoring and engagement of pathogens.

Spleen and Thymus

The spleen filters blood, removing dead cells and foreign bodies while acting as a blood reservoir. The thymus is critical for T cell maturation and also influences immune response through hormone production.

Importance of the Thymus

As the body matures, the thymus gradually decreases in size. Its educational role ensures T cells that could harm the body (auto-reactive) are eliminated, while those potentially beneficial for immune defense are retained.