MEET THE LYMPHOCYTES
What are the primary lymphoid organs where T cells mature and mammalian B cells develop, respectively?
Answer: Thymus and bone marrow.
Explanation: The thymus is where T lymphocytes mature, while B lymphocytes develop in the bone marrow of mammals. In birds, B cells develop in the bursa of Fabricius. These are primary lymphoid organs, where lymphocytes develop and acquire their antigen receptors before encountering their specific antigens.
Which region of an antibody binds directly to a specific antigen?
Answer: Fab region.
Explanation: The Fab region contains the antigen-binding portion of the antibody and determines antigen specificity. The Fc region is responsible for interactions with immune effector mechanisms, such as binding Fc receptors on phagocytes and participating in complement activation.
Remember: Fab = antigen binding; Fc = effector functions.
Which antibody class predominates in mucosal secretions and prevents microbes from attaching to epithelial surfaces?
Answer: IgA.
Explanation: Secretory IgA is the major antibody involved in adaptive mucosal protection. It is found in secretions such as saliva, tears, intestinal fluid, and milk. Its major function is preventing pathogen adherence to epithelial surfaces, a mechanism called immune exclusion.
What term describes the change from undetectable to detectable antigen-specific antibodies in an animal's serum?
Answer: Seroconversion.
Explanation: Seroconversion occurs when an animal develops detectable antibodies against a particular antigen. This is important because infection can occur before antibodies become detectable. An early negative antibody test therefore does not necessarily exclude infection.
Which preformed inflammatory mediator, released primarily by mast cells, rapidly increases vascular permeability and contributes to vasodilation?
Answer: Histamine.
Explanation: Histamine is stored in mast-cell granules and can be released rapidly following stimulation. It causes vasodilation and increased vascular permeability, contributing to redness, warmth, and swelling during acute inflammation.
Because histamine is preformed, it acts much faster than mediators that require new synthesis.
Which lymphoid organs filter lymph and provide sites where lymphocytes encounter antigens?
Answer: Lymph nodes.
Explanation: Lymph nodes are secondary lymphoid organs that filter lymph and allow antigens, antigen-presenting cells, and lymphocytes to interact. This provides an opportunity for naïve lymphocytes to recognize their specific antigens and become activated.
What two cell types can result from B-cell activation, providing immediate antibody production and future immune memory?
Answer: Plasma cells and memory B cells.
Explanation: Activated B lymphocytes undergo clonal expansion and differentiate into plasma cells, which secrete antibodies, and memory B cells, which contribute to faster and stronger responses upon subsequent exposure to the same antigen.
Which two adaptive immune defenses are especially important for extracellular microbes and virus-infected host cells, respectively?
Answer: Antibodies and cytotoxic T lymphocytes (CTLs).
Explanation: Antibodies protect against extracellular pathogens through neutralization, opsonization, and complement activation. CTLs recognize and kill infected host cells, making them important for infections involving intracellular pathogens such as viruses.
Compared with a primary response, a secondary humoral response develops more rapidly and is dominated by which antibody class?
Answer: IgG.
Explanation: A primary antibody response typically takes approximately 7–10 days to become detectable and is initially dominated by IgM. A secondary response can become detectable in approximately 1–2 days, is generally larger, and is dominated by IgG.
Which inflammatory mediators direct leukocytes toward sites of infection by establishing chemical signals for migration?
Answer: Chemokines.
Explanation: Chemokines are signaling proteins that help guide leukocytes toward areas of inflammation. They play an important role in leukocyte recruitment and chemotaxis.
Unlike histamine, which primarily contributes to early vascular changes, chemokines are especially important for directing immune-cell movement.
Which cytokine is produced by Th1 helper T cells to activate macrophages against intracellular pathogens?
Answer: Interferon-gamma (IFN-γ).
Explanation: Th1 cells produce IFN-γ, which activates macrophages and enhances their ability to kill microbes that survive inside them, such as Mycobacterium bovis. The Th1 cell coordinates the response through cytokine production, while the activated macrophage performs microbial killing.
Which antibody classes are particularly associated with mucosal protection and an early primary humoral response, respectively?
Answer: IgA and IgM.
Explanation: IgA is the dominant antibody class at mucosal surfaces and helps prevent pathogens from attaching to epithelial cells. IgM is typically prominent during the primary antibody response, especially early after first exposure. IgG becomes more prominent in secondary responses.
A calf receives protective maternal antibodies through colostrum. What type of immunity does this represent?
Answer: Naturally acquired passive immunity.
Explanation: The calf receives antibodies produced by its mother rather than generating them itself. Passive immunity provides immediate protection but is temporary because the transferred antibodies are not continuously replenished.
A dog's antibody titer increases from 1:8 to 1:32 between acute and convalescent samples. What magnitude of increase does this represent, and why is it significant?
Answer: A fourfold rise in antibody titer.
Explanation: The titer increased from 8 to 32, representing a fourfold increase. The serology lecture explains that a fourfold or greater increase between acute and convalescent samples supports active infection when interpreted alongside clinical findings.
Samples are generally collected 2–3 weeks apart and tested in parallel.
An inflammatory response initially protects an animal but begins damaging healthy tissue when mediator activity persists. Why must inflammatory mediators normally be rapidly inactivated and tightly regulated?
Answer: To prevent excessive inflammation and collateral tissue injury.
Explanation: Inflammatory mediators help coordinate host defense, but uncontrolled or prolonged activity can damage healthy tissues.
The slides describe regulatory mechanisms such as rapid mediator degradation, antioxidant defenses, endogenous inhibitors, and anti-inflammatory or pro-resolving signals.
The goal is to control inflammation without completely eliminating its protective effects.
A cow has been exposed to Mycobacterium bovis. A delayed skin reaction following tuberculin injection indicates what type of adaptive immune response?
Answer: Delayed-type hypersensitivity (DTH), a cell-mediated immune response.
Explanation: DTH is an antigen-specific, cell-mediated response involving Th1 cells, IFN-γ, and macrophage activation. Previously sensitized T cells recognize the test antigens and release cytokines that promote a local inflammatory reaction. The tuberculosis skin test evaluates this response rather than measuring circulating antibodies.
Antibodies coat a bacterial surface, allowing phagocytes to recognize the antibody's Fc region and ingest the bacterium more effectively. What is this process called?
Answer: Opsonization.
Explanation: Opsonization involves coating microbes with molecules that facilitate phagocytosis. The Fab region of an antibody binds the pathogen, while the Fc region interacts with receptors on phagocytes, making the organism easier to recognize and engulf.
A pathogen survives and replicates inside macrophages. Which helper T-cell subtype and major cytokine are important for activating macrophages to kill the pathogen?
Answer: Th1 cells and IFN-γ.
Explanation: Th1 cells release IFN-γ, which activates macrophages and enhances their ability to destroy intracellular microbes. This is an important mechanism of cell-mediated immunity and delayed-type hypersensitivity, particularly for pathogens such as Mycobacterium.
A horse has a positive antibody titer on a single serum sample but no current clinical signs. Why can't this result independently establish an active infection?
Answer: Antibodies may reflect previous exposure, vaccination, or cross-reactivity rather than current infection.
Explanation: A positive antibody titer indicates that antibodies recognizing the test antigen are present. However, antibodies can persist after an infection has resolved or following vaccination. Cross-reacting antibodies can also produce positive results.
This is why serology must be interpreted alongside history, clinical findings, and sometimes paired serum samples.
A sheep flock experiences abortions during lambing. Placental smear examination identifies organisms compatible with Chlamydia abortus, and PCR confirms the organism. What is the diagnosis?
Answer: Enzootic abortion of ewes (EAE), caused by Chlamydia abortus.
Explanation: In the Happy Heel Farm case, the diagnosis is established by combining the flock history, clinical findings, placental smear examination, and confirmatory PCR.
This demonstrates the importance of a systematic flock investigation rather than relying on one finding.
A cow's blood is stimulated with M. bovis antigens, and antigen-specific T cells release IFN-γ. Which arm of adaptive immunity is being measured, and what does a positive result indicate?
Answer: Cell-mediated immunity; a positive result indicates an antigen-specific immune response to the test antigens.
Explanation: The interferon-gamma release assay detects IFN-γ produced by sensitized T cells after stimulation with tuberculosis antigens. It evaluates cell-mediated immunity rather than humoral immunity. A positive response provides evidence of immune recognition but does not, by itself, establish that the animal has clinical disease.
A laboratory requires antibodies that all recognize the same epitope rather than a mixture recognizing multiple epitopes. What type of antibody preparation should be selected?
Answer: Monoclonal antibodies.
Explanation: Monoclonal antibodies originate from a single B-cell clone and recognize the same epitope. Polyclonal antibodies are produced by multiple B-cell clones and may recognize different epitopes on an antigen. Monoclonal preparations are useful when consistent, specific recognition is needed in diagnostic testing.
A pathogen avoids antibodies by moving rapidly between host cells and spending little time extracellularly. What type of adaptive immune response becomes especially important for controlling infection?
Answer: Cell-mediated immunity, particularly CTL-mediated killing of infected cells.
Explanation: Antibodies are most effective against extracellular targets. Pathogens that remain inside host cells are less accessible to circulating antibodies.
A veterinarian suspects an infectious disease, but an animal's serum is antibody-negative shortly after suspected exposure. Why can't this result rule out infection?
Answer: The animal may have been tested before producing detectable antibodies.
Explanation: During the early period following infection, an animal may not yet have undergone seroconversion. Antibody concentrations may therefore be below the test's detection threshold.
A negative result early in infection does not necessarily rule out disease, and repeat testing or another diagnostic method may be necessary.
A sheep flock has stable positive antibody titers in several ewes, while others show a fourfold rise over three weeks. Which animals have stronger evidence of recent infection?
Answer: The ewes with a fourfold rise in paired antibody titers.
Explanation: A significant increase in antibody titers over time supports recent infection or immune stimulation.
Stable positive titers may reflect previous exposure, so they do not establish when infection occurred.