How Your Body Fights Infection

Posted on Dec 2, 2024 in Biology

Nonspecific Defenses

These defenses act rapidly and comprise four types:

1. Inflammation

If passive barriers are breached, inflammation occurs, activating other responses like pain, warmth, and swelling in the affected area. The process is as follows:

  1. Entrance of a stimulus
  2. Affected cells release inflammatory mediators (histamine, bradykinin, prostaglandins, etc.)
  3. These mediators act on the affected tissue, causing increased white blood cell count, vasodilation, increased capillary permeability, and activation of phagocytes.
  4. Consequences include chemotaxis, increased leukocyte defensive actions, increased blood flow to the affected area, and a rise in defensive elements (leukocytes, complement molecules, and antibodies). Increased capillary permeability allows phagocytes, antibodies, complement, and fibrinogen to enter infected tissues, facilitating phagocyte activation and chemotaxis.

2. Phagocytes

These white blood cells are of two types:

  1. Myeloid series: Formed in bone marrow, exhibit amoeboid movement, and include granulocytes (PMNs) and monocytes.
  2. Lymphoid series (lymphocytes): Do not phagocytose but produce specific antibodies.

The primary function of phagocytes is phagocytosis, with histocytes acting first, followed by neutrophils and then macrophages (which have the highest phagocytic capacity). The process involves four stages:

  1. Attachment to the element via opsonins
  2. Ingestion
  3. Death and digestion through enzyme release
  4. Expulsion of undigested remains

3. Complement

This system of plasma proteins acts rapidly in defense with three key functions:

  1. Mediates inflammation
  2. Opsonizes foreign cells to facilitate phagocyte and antibody action
  3. Causes lysis of invasive cells through plasma membrane rupture

Complement activation can occur in two ways:

  1. Classical pathway: Triggered by antibodies bound to antigens
  2. Alternative pathway: Antibody-independent, triggered by complement components binding to bacterial surface polysaccharides

4. Interferon

Virus-infected cells produce interferon, a protein that prevents infection spread. It has two main actions:

  1. Inhibits viral replication in infected cells not yet destroyed
  2. Activates NK cells to eliminate virus-infected or cancerous cells

Humoral Response and Immunological Memory

Humoral immunity involves specific immune mechanisms using proteins (antibodies) produced against foreign antigens. These antibodies circulate in blood, lymph, and interstitial fluids.

B Lymphocytes

B lymphocytes differentiate in the bone marrow and acquire the ability to produce antibodies against specific invader antigens. Millions of different B lymphocytes produce different antibodies, covering a vast range of antigens. This diversity arises from DNA fragmentation and rearrangement of antibody segments. When an antigen encounters a B lymphocyte with a matching antibody, it binds to the antibody on the lymphocyte’s membrane. The strength of this binding determines the immune response intensity. Upon antigen recognition, B lymphocytes activate and divide rapidly, forming clones. Some activated B lymphocytes become plasma cells, while others remain as memory cells. Plasma cells have a limited lifespan, while memory cells provide a reserve for future antigen encounters.

Antibodies

Antibodies are proteins with a small carbohydrate component. Each antibody molecule consists of four polypeptide chains: two heavy and two light. There are five classes of antibodies (IgG, IgA, IgM, IgD, and IgE) with different sequences. Each antibody has a constant region for binding to B lymphocytes, phagocytes, or complement, and a variable region specific to the antigen. Each antibody molecule can bind two antigen molecules.

Types of Antibodies

Antibodies are categorized by their polypeptide chain composition:

  1. IgG: Monomeric, found in blood and extracellular fluids
  2. IgA: Monomeric or dimeric, found in blood, extracellular fluids, saliva, tears, etc.
  3. IgM: Pentameric, found in blood, extracellular fluids, and on B lymphocyte surfaces
  4. IgD: Monomeric, found on B lymphocyte membranes
  5. IgE: Monomeric, found in blood and extracellular fluids

Functions of Antibodies

Antibodies have direct effects:

  1. Neutralization: Antibody binding eliminates the antigen’s negative effects
  2. Precipitation: Antibodies bind to multivalent antigens, forming large aggregates
  3. Agglutination: Antibodies cross-link antigens on cell surfaces

They also have indirect effects, such as opsonization and complement activation via the classical pathway.

Immunological Memory

Upon encountering an antigen, the body stores information, leading to the formation of memory B lymphocytes. These cells persist even after the antigen is eliminated, enabling a faster and more efficient response upon subsequent encounters. This memory can eliminate antigens before symptoms develop, as seen in vaccination.