Lymphatic and immune systems

Lymphatic system

The Lymphatic system has several crucial roles:

• Equalization of fluid distribution
• Transport of proteins and large glycerides
• Production of lymphocytes for immune defense,
• The return of materials to the blood.
  It accomplishes these functions using one‐way lymphatic capillaries, which open their flaps to allow interstitial fluid in when local pressure is higher than inside the vessel, and then close to prevent backflow when the pressure reverses.

One specialized form of these capillaries, the lacteal, absorbs dietary fats in the small intestine, ensuring large lipids enter lymphatic circulation rather than directly into the bloodstream. The system also helps return any plasma protein that leaks out of blood capillaries, maintaining proper osmotic balance.

Although lymphocytes originate in the bone marrow, lymphoid tissues such as the thymus (where T cells mature) and various lymph node clusters are where these cells reside, proliferate, and differentiate. As white blood cells within lymph nodes filter pathogens and debris, they can become activated and launch an immune response—including antibody production—to protect the body.

Composed mainly of water, proteins, and immune cells, lymph closely resembles blood plasma. It forms when fluid leaves the capillaries, becomes interstitial fluid, and then diffuses into lymphatic vessels.
Eventually, it merges with the veins, returning these fluids and proteins to the bloodstream. Within the nodes, if foreign antigens are detected, resident lymphocytes are triggered to divide and produce defensive molecules, ensuring that any leaked cells and proteins are not only recovered but also screened for potential threats before being returned to circulation.

Immune system

Innate (non-specific) vs. adaptive (specific) immunity

• The Immune system defends the body through both innate immunity and adaptive immunity. Innate immunity provides a rapid, general response to anything foreign, while adaptive immunity mounts a highly specific defense against particular pathogens or antigens.

Several types of white blood cells coordinate these responses:

• Macrophages engulf microbes and present portions of their antigens to other immune cells, linking innate and adaptive systems.
• Neutrophils are fast responders that ingest and destroy pathogens.
• Mast cells release histamine during allergic reactions, increasing inflammation.
• Natural killer cells target infected or abnormal cells and induce their destruction.
• Dendritic cells excel at capturing pathogens and displaying their antigens to activate T cells. Lymphocytes
  • Among lymphocytes, T-lymphocytes mature in the thymus and include cytotoxic T cells, which trigger apoptosis in infected cells, and helper T cells, which stimulate macrophages, T cells, and B cells.
  • Meanwhile, B-lymphocytes mature in the bone marrow; upon encountering a pathogen, they can differentiate into plasma cells that secrete antibodies or memory cells that enable a faster response if the same pathogen reappears.

Immune tissues

Immune tissues support these varieties of white blood cells:

• The spleen filters pathogens and old blood cells from circulation.
• Lymph nodes remove pathogens from lymph and house lymphocytes that scan for foreign antigens. When antigens are detected, an immune response initiates, involving inflammation, increased phagocytosis, and, if necessary, production of pathogen-specific antibodies.
• The thymus and bone marrow are also considered immune tissue, as they produce T and B cells.

In innate immunity, physical barriers like skin and mucus membranes prevent pathogen entry, while cells such as phagocytes and natural killer cells, along with antimicrobial proteins, attack invaders non-specifically. The inflammatory response recruits white blood cells to infection sites, aided by fever, which heightens their activity.

By contrast, adaptive immunity focuses on distinct pathogens. Antigen-presenting cells showcase a pathogen’s foreign markers (antigen) to T and B cells, triggering cytotoxic T cells to kill infected cells and helper T cells to activate other immune cells. B cells produce antibodies, Y-shaped proteins that bind antigens for neutralization, opsonization (making them easier for phagocytes to engulf), or complement activation (disrupting pathogen membranes). Over time, memory cells remain to mount a faster, more potent response upon re-exposure, explaining why secondary infections are often milder than the first.

Antigens and antibodies

An antibody’s structure contains two light chains and two heavy chains, joined by disulfide bonds. Each tip of its Y shape has a hypervariable region attuned to a unique antigen, ensuring precise binding.

When a pathogen enters an antigen-presenting cell (APC), it is broken down into fragments, or antigen, and displayed on the cell’s surface. These antigen fragments are recognized by a T cell receptor, triggering immune responses tailored to how and where the pathogen is found:

• In an extracellular pathogen scenario, a macrophage engulfs the invader and presents antigen on its surface. A helper T cell recognizes this antigen and signals the macrophage to destroy more of the pathogen, while also activating B cells to produce antibodies.
• If it is an intracellular pathogen, the infected host cell displays antigen fragments on its surface. A cytotoxic T cell detects these fragments and instructs the host cell to self-destruct, eliminating the threat before it can spread.

Clonal selection describes the adaptive immune process in which only the small subset of B or T lymphocytes that specifically recognize a particular antigen multiply into large clones. Once activated by their matching antigen, these lymphocytes proliferate and differentiate into effector cells (such as plasma cells that secrete antibodies) or memory cells that provide long-lasting immunity.

Recognition of self vs. non-self ensures the immune system distinguishes the body’s own tissues from foreign substances. During lymphocyte development, cells that react strongly against the body’s own antigens are usually eliminated or inactivated. When this process fails, autoimmune diseases can arise, with the immune system attacking self-tissues and causing conditions like rheumatoid arthritis or type 1 diabetes.

Finally, the major histocompatibility complex (MHC) is crucial for presenting peptide fragments to T cells. Class I MHC molecules, found on nearly all nucleated cells, present intracellular peptides to cytotoxic T cells, while Class II MHC molecules, restricted to professional antigen-presenting cells (e.g., macrophages, dendritic cells, B cells), display extracellular peptide fragments to helper T cells. This presentation system orchestrates the adaptive immune response by ensuring T cells detect and respond appropriately to foreign antigens.