2: The Lymphatic and Immune System

The Lymphatic System: Structure and Basic Functions

Outlines lymphatic vessels, ducts, and trunks; movement of lymph; and the roles of the lymphatic system in fluid balance, lipid absorption, and immune cell transport.

Lymphoid Organs: Primary and Secondary

Defines and contrasts primary lymphoid organs (bone marrow, thymus) where lymphocytes develop and mature, with secondary lymphoid organs (lymph nodes, spleen, MALT = Mucosa Associated Lymphatic Tissue) where immune responses are initiated. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.

Innate Immunity: The First Line of Defense

Physical and chemical barriers are the body’s first line of defense against infection and are part of innate immunity. Skin acts as a tough physical barrier. Its outer keratinized layer prevents most microbes from entering, and its slightly acidic pH plus antimicrobial peptides (like defensins) inhibit microbial growth on the surface. Intact skin is one of the most effective protections against infection. Mucous membranes lining the respiratory, gastrointestinal, and genitourinary tracts trap microbes in mucus. Structures like cilia in the airways move mucus (and trapped particles) out of the body. Stomach acidity provides a strong chemical barrier. Gastric acid has a very low pH (around 1–3), which destroys many ingested pathogens before they can reach the intestines. Additional chemical barriers include lysozyme in tears and saliva (which breaks down bacterial cell walls) and the slightly acidic environment of the vagina, which inhibits many pathogens.

The first line of physiological defense explains barrier defenses and innate responses, including phagocytes, natural killer cells, complement, interferons, and inflammation, emphasizing that innate immunity provides rapid, non-specific protection and generates signals (cytokines, co-stimulatory molecules, antigen presentation) that activate and shape the adaptive immune response.

Adaptive Immunity: Specific, Memory-Based, and Shaped by Innate Signals

The adaptive immune response is a slow-acting, highly specific, and long-lasting defense system that targets specific pathogens and creates lasting memory. Key characteristics include specificity (tailored to recognize specific pieces of a pathogen called antigens); memory (remembers past infections to fight future exposures faster and delay (takes days to activate fully during a first-time infection).

Understanding adaptive immunity does indeed start with two key definitions:

• Antigen: An antigen is any molecule that can be specifically recognized by the receptors of B cells or T cells and can trigger an adaptive immune response. Antigens are often proteins or large polysaccharides (for example, components of bacteria, viruses, or pollen), but can also include other molecules. A small part of the antigen that is actually recognized by an antibody or T cell receptor is called an epitope.
• Antibody: An antibody (also called an immunoglobulin, Ig) is a Y-shaped protein produced by activated B cells (plasma cells) in response to an antigen. Antibodies bind specifically to their matching antigen/epitope. Once bound, they help neutralize pathogens, mark them for destruction (opsonization), or activate other immune components such as complement.

Main players (Lymphocytes) are B Cells (mature in the bone marrow and drive humoral immunity by making antibodies. and T Cells (mature in the thymus and drive cell-mediated immunity. There are several functional types of T lymphocytes, the major ones being Helper T Cells (CD4⁺): coordinate the immune response and activate other cells. and Cytotoxic T Cells (CD8⁺): identify and destroy infected or cancerous host cells. Two Types of Responses:

• Humoral/ Antibody-mediated Response: B cells turn into plasma cells that release Y-shaped antibodies into the blood to tag and neutralize floating pathogens.
• Cell-Mediated Response: T cells directly attack infected cells or signal other cells to destroy the threat.

Immunological Memory, Pathogen Targets, and Evasion

Early childhood is a time when the body develops much of its immunological memory that protects it from diseases in adulthood. The components of the immune response that have the maximum effectiveness against a pathogen are often associated with the class of pathogen involved. Bacteria and fungi are especially susceptible to damage by complement proteins, whereas viruses are taken care of by interferons and cytotoxic T cells. Worms are attacked by eosinophils. Pathogens have shown the ability, however, to evade the body’s immune responses, some leading to chronic infections or even death. The immune system and pathogens are in a slow, evolutionary race to see who stays on top. Modern medicine, hopefully, will keep the results skewed in humans’ favor.

Immune Dysregulation: Immunodeficiency, Hypersensitivity, and Autoimmunity

The immune response can be under-reactive or over-reactive. Suppressed immunity can result from inherited genetic defects or by acquiring viruses. Over-reactive immune responses include the hypersensitivities: B cell- and T cell-mediated immune responses designed to control pathogens, but that lead to symptoms or medical complications. The worst cases of over-reactive immune responses are autoimmune diseases, where an individual’s immune system attacks their own body because of the breakdown of immunological tolerance. These diseases are more common in the aged, so treating them will be a challenge in the future as the aged population in the world increases.

Transfusion, Transplantation, and Cancer Immunology

Blood transfusion and organ transplantation both require an understanding of the immune response to prevent medical complications. Blood needs to be typed so that natural antibodies against mismatched blood will not destroy it, causing more harm than good to the recipient. Transplanted organs must be matched by their MHC molecules and, with the use of immunosuppressive drugs, can be successful even if an exact tissue match cannot be made. Another aspect to the immune response is its ability to control and eradicate cancer. Although this has been shown to occur with some rare cancers and those caused by known viruses, the normal immune response to most cancers is not sufficient to control cancer growth. Thus, cancer vaccines designed to enhance these immune responses show promise for certain types of cancer.

Opportunistic Infection as a Clue to Immune Failure

Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible. The five patients from LA, though, were between 29 and 36 years of age and should have been in the prime of their lives, immunologically speaking. What could be going on?

HIV and AIDS: A New Disease Highlighting Immune Function

Acquired immune deficiency syndrome, or AIDS, turned out to be a condition caused by the previously unknown human immunodeficiency virus (HIV). Although nearly 100 percent fatal in those with active HIV infections in the early years, the development of anti-HIV drugs has transformed HIV infection into a chronic, manageable disease and not the certain death sentence it once was. One positive outcome resulting from the emergence of HIV disease was that the public’s attention became focused as never before on the importance of having a functional and healthy immune system.

Thumbnail: Antibodies Tag Pathogens for Phagocytosis. (CC BY 4.0; via Antigen-Antibody Interactions).