BCSE Veterinary Immunology — Questions and Answers
Question 1: Which immunoglobulin class is the predominant antibody in colostrum and provides passive mucosal immunity to the neonatal gastrointestinal tract?
- IgM
- IgG
- IgA (Correct answer)
- IgE
Correct answer: IgA
Secretory IgA (sIgA) is the dominant immunoglobulin in colostrum and mucosal secretions. It is transported across epithelial cells via the polymeric immunoglobulin receptor (pIgR) and resists proteolytic digestion in the gut lumen, making it critical for neonatal mucosal defense. IgG predominates in serum; IgM is the first antibody produced in primary responses; IgE is involved in allergic reactions.
Question 2: Type III hypersensitivity reactions are mediated by:
- IgE cross-linking mast cell FcεRI receptors
- Cytotoxic T lymphocytes directly killing target cells
- Immune complex deposition activating complement (Correct answer)
- IgG or IgM binding cell-surface antigens and activating complement or ADCC
Correct answer: Immune complex deposition activating complement
Type III (immune complex) hypersensitivity occurs when antigen-antibody complexes deposit in vessel walls or tissues, activating complement (especially C3a and C5a), recruiting neutrophils, and causing local inflammation. Equine recurrent uveitis and bovine hypersensitivity pneumonitis are veterinary examples. Type I = IgE/mast cells; Type II = antibody against cell-surface antigens; Type IV = T-cell mediated.
Question 3: Which cell type is the primary antigen-presenting cell responsible for activating naïve T lymphocytes?
- Natural killer cells
- Dendritic cells (Correct answer)
- Plasma cells
- Mast cells
Correct answer: Dendritic cells
Dendritic cells are the most potent professional antigen-presenting cells (APCs) for priming naïve T cells. They engulf antigens in peripheral tissues, migrate to lymph nodes, and present peptides on MHC class I and II molecules along with costimulatory signals (CD80/CD86). Macrophages and B cells can also act as APCs but are less efficient at activating naïve T cells. NK cells lack antigen-presenting function; plasma cells are terminally differentiated B cells.
Question 4: Failure of passive transfer (FPT) in foals is defined as a serum IgG concentration below what threshold at 24 hours of age?
- 200 mg/dL
- 400 mg/dL
- 800 mg/dL (Correct answer)
- 1200 mg/dL
Correct answer: 800 mg/dL
FPT in foals is defined as serum IgG < 800 mg/dL (8 g/L) at 18–24 hours of age. Foals are agammaglobulinemic at birth because equine epitheliochorial placentation prevents in-utero transfer of maternal antibodies; they depend entirely on colostral absorption during the first 12–24 hours before gut closure. Levels 400–800 mg/dL represent partial FPT.
Question 5: Modified live vaccines (MLV) confer longer-lasting immunity than killed vaccines primarily because they:
- Contain higher doses of antigen per dose
- Require adjuvant to activate antigen-presenting cells
- Replicate in the host, producing endogenous antigen that activates both CD4+ and CD8+ T cells (Correct answer)
- Stimulate only humoral immunity without cellular involvement
Correct answer: Replicate in the host, producing endogenous antigen that activates both CD4+ and CD8+ T cells
MLV vaccines replicate (to a limited extent) within host cells, generating endogenous antigen that enters the MHC class I pathway, activating cytotoxic CD8+ T cells in addition to CD4+ helper T cells and B cells. This mirrors a natural infection and generates robust, durable immunity. Killed/inactivated vaccines primarily stimulate humoral immunity via MHC class II and typically require adjuvants and booster doses.
Question 6: In canine autoimmune hemolytic anemia (AIHA), erythrocyte destruction occurs primarily via which mechanism?
- Type I hypersensitivity degranulation of basophils
- Type II hypersensitivity — IgG/IgM antibodies against RBC surface antigens leading to complement lysis and phagocytosis (Correct answer)
- Type III hypersensitivity immune complex deposition in splenic sinusoids
- Type IV hypersensitivity cytotoxic T-cell attack on erythrocytes
Correct answer: Type II hypersensitivity — IgG/IgM antibodies against RBC surface antigens leading to complement lysis and phagocytosis
AIHA is a classic Type II hypersensitivity. Auto-antibodies (typically IgG) bind erythrocyte surface antigens; IgM can also fix complement directly. Opsonized RBCs are phagocytosed by splenic and hepatic macrophages (extravascular hemolysis) or lysed by the membrane attack complex (intravascular hemolysis). Spherocytes on blood smear and a positive Coombs test are hallmarks.
BCSE Veterinary Immunology
Which immunoglobulin class is the predominant antibody in colostrum and provides passive mucosal immunity to the neonatal gastrointestinal tract?