The Immune System in Depth

• A. Complement proteins
• B. Intact skin ✓
• C. Natural killer cells
• D. Interferons

Why: Intact skin forms a physical barrier that prevents most pathogens from entering the body. Complement proteins, natural killer cells, and interferons are components of the innate immune response but are not physical barriers.

• A. B lymphocytes
• B. Neutrophils ✓
• C. Helper T cells
• D. Plasma cells

Why: Neutrophils are the most abundant white blood cells and are typically the first responders to infection, rapidly engulfing and destroying bacteria through phagocytosis. B lymphocytes, helper T cells, and plasma cells are part of the adaptive immune system.

• A. Opsonization
• B. Apoptosis
• C. Phagocytosis ✓
• D. Chemotaxis

Why: Phagocytosis is the process where cells such as macrophages and neutrophils engulf and digest pathogens or debris. Opsonization is the coating of a pathogen to enhance phagocytosis, apoptosis is programmed cell death, and chemotaxis is movement toward a chemical signal.

• A. Antibodies
• B. Histamines
• C. Interferons ✓
• D. Perforins

Why: Interferons are signaling proteins released by virus-infected cells that alert neighboring cells and induce antiviral defenses. Antibodies are adaptive immune molecules, while histamines and perforins have different roles in inflammation and cell killing respectively.

• A. To permanently remove all memory of a pathogen
• B. To produce long-lasting antibodies against a pathogen
• C. To isolate injured or infected tissue and recruit immune cells to the site ✓
• D. To replace damaged skin cells with stem cells

Why: The inflammatory response increases blood flow and vessel permeability, isolates the damaged area, and recruits immune cells such as neutrophils and macrophages to fight infection and begin tissue repair. It does not create immunological memory or antibodies, which are features of adaptive immunity.

• A. Neutrophils
• B. B lymphocytes (B cells) ✓
• C. Eosinophils
• D. Platelets

Why: B lymphocytes mature into plasma cells that secrete antibodies, which are central to adaptive immune responses. Neutrophils and eosinophils are innate immune cells, and platelets are involved in clotting rather than immunity.

• A. A type of white blood cell that engulfs pathogens
• B. A molecule that the immune system can recognize and trigger a response against ✓
• C. A protein that circulates in the blood to fight infection
• D. A chemical signal that recruits immune cells to inflammation

Why: An antigen is any molecule, often a protein or polysaccharide on a pathogen's surface, that lymphocyte receptors can bind to, initiating an adaptive immune response. The other options describe cells or signaling molecules instead.

• A. Bone marrow only
• B. Spleen
• C. Thymus ✓
• D. Liver

Why: T cells receive their name from the thymus, where they mature and learn to distinguish self from non-self antigens. B cells mature in the bone marrow.

• A. Adaptive immunity responds identically to every pathogen
• B. Adaptive immunity is faster but less specific
• C. Adaptive immunity is highly specific and develops immunological memory ✓
• D. Adaptive immunity relies only on physical barriers like skin

Why: Adaptive immunity tailors its response to specific antigens and retains memory of past encounters, allowing faster, stronger responses on re-exposure. Innate immunity responds more generically and more quickly.

• A. To immediately destroy pathogens on first contact
• B. To remove dead cells from tissues
• C. To provide a faster and stronger response upon re-exposure to the same antigen ✓
• D. To produce hormones that regulate inflammation

Why: Memory cells persist long after an infection resolves. When the same antigen reappears, they respond more quickly and vigorously, which is the basis of long-lasting immunity and vaccine effectiveness.

• A. MHC class I ✓
• B. MHC class II
• C. CD4 co-receptor
• D. Immunoglobulin M

Why: MHC class I molecules are expressed on nearly all nucleated cells and display peptides derived from proteins made inside the cell, such as viral proteins. These complexes are recognized by CD8+ cytotoxic T cells.

• A. Red blood cells
• B. Platelets
• C. Dendritic cells ✓
• D. Neutrophils

Why: Dendritic cells are the most potent professional antigen-presenting cells. They capture antigens in peripheral tissues, migrate to lymph nodes, and efficiently prime naive T cells. Macrophages and B cells also qualify, but dendritic cells are the most effective.

• A. CD8
• B. CD28
• C. CD4 itself binds MHC class II ✓
• D. CD19

Why: CD4 is the co-receptor on helper T cells that binds to the non-polymorphic region of MHC class II molecules, stabilizing the interaction with the T cell receptor. CD8 performs the analogous role for MHC class I on cytotoxic T cells.

• A. Antigen recognition via TCR and co-stimulation via CD28 binding B7 molecules ✓
• B. Antibody binding and complement activation
• C. Cytokine release and apoptosis
• D. MHC class I binding and NK cell killing

Why: Signal 1 is TCR recognition of peptide-MHC complexes, and Signal 2 is co-stimulation, most importantly CD28 on the T cell binding B7 (CD80/CD86) on the antigen-presenting cell. Without the second signal, T cells become anergic rather than activated.

• A. Cross-presentation in the cytosol
• B. Degradation of antigens in endosomes and lysosomes followed by loading in the MIIC compartment ✓
• C. Direct binding of intact proteins to the cell surface
• D. Proteasomal degradation followed by TAP transport into the ER

Why: Extracellular antigens are taken up by endocytosis, degraded in acidic endosomal and lysosomal compartments, and the resulting peptides are loaded onto MHC class II in specialized MIIC compartments. The invariant chain protects the MHC II groove until loading occurs.

• A. Directly killing virus-infected cells
• B. Producing antibodies that bind to specific antigens ✓
• C. Engulfing and digesting bacteria
• D. Releasing histamine during allergic reactions

Why: B cells produce antibodies, which are proteins that recognize and bind to specific antigens, marking pathogens for destruction or neutralizing them. Killing infected cells is mainly the job of cytotoxic T cells, and engulfing bacteria is typically done by phagocytes such as macrophages and neutrophils.

• A. Thymus
• B. Spleen
• C. Bone marrow ✓
• D. Liver

Why: B cells develop and mature in the bone marrow. The thymus is where T cells mature, which is why T cells are named for it, while the spleen and lymph nodes are sites where mature B cells encounter antigens and become activated.

• A. Lipids
• B. Carbohydrates
• C. Nucleic acids
• D. Proteins ✓

Why: Antibodies, also called immunoglobulins, are proteins. Each has a Y-shaped structure made of heavy and light polypeptide chains, with variable regions at the tips that give each antibody its antigen-binding specificity.

• A. It becomes a red blood cell
• B. It proliferates and differentiates into plasma cells and memory B cells ✓
• C. It is permanently inactivated
• D. It converts into a T cell

Why: Activation of a B cell by its matching antigen, often with signals from helper T cells, triggers clonal expansion. The cells then differentiate into antibody-secreting plasma cells that fight the current infection and long-lived memory B cells that enable a faster response to future exposures.

• A. IgE
• B. IgA
• C. IgM ✓
• D. IgD

Why: IgM is the first antibody class produced during a primary immune response. It forms pentamers that are highly effective at activating complement. Later in the response, B cells typically switch to producing IgG, IgA, or IgE, which have more specialized functions.

• A. To produce antibodies against specific antigens
• B. To enhance pathogen clearance through opsonization, inflammation, and direct lysis of microbes ✓
• C. To store memory of past infections
• D. To regulate blood clotting factors only

Why: The complement system is a group of plasma proteins that helps clear pathogens by coating them for phagocytosis (opsonization), recruiting inflammatory cells, and forming membrane attack complexes that lyse certain microbes.

• A. Alternative pathway
• B. Lectin pathway
• C. Classical pathway ✓
• D. Terminal pathway

Why: The classical pathway is triggered when C1q binds to antibodies (mainly IgG or IgM) bound to antigens, making it the antibody-dependent route of complement activation.

• A. They directly kill bacteria by forming pores
• B. They promote inflammation by recruiting and activating neutrophils and mast cells ✓
• C. They inhibit the production of cytokines
• D. They break down complement proteins

Why: C3a and C5a are small peptide fragments released during complement activation that trigger inflammation, causing mast cell degranulation, increased vascular permeability, and chemotaxis of immune cells.

• A. C1q and C4b
• B. C3b and factor B
• C. C5b combined with C6, C7, C8, and multiple C9 molecules ✓
• D. IgG and C reactive protein

Why: The membrane attack complex forms when C5b binds C6, C7, and C8, followed by polymerization of several C9 molecules, creating a pore that disrupts the membranes of target cells such as certain bacteria.

• A. Pallor and coldness
• B. Rubor (redness) and calor (heat) ✓
• C. Atrophy and fibrosis
• D. Lymphocyte predominance

Why: The classic signs of acute inflammation are rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function), reflecting increased blood flow and vascular changes at the site of injury.

• A. Somatic hypermutation in the bone marrow
• B. Negative selection in the thymus ✓
• C. Class switching in lymph nodes
• D. Complement-mediated lysis in the spleen

Why: Negative selection in the thymus eliminates T cells that bind strongly to self-antigens presented on MHC molecules, preventing many autoreactive T cells from leaving the thymus.

• A. Natural killer cells
• B. Neutrophils
• C. Regulatory T cells (Tregs) ✓
• D. Mast cells

Why: Regulatory T cells, often marked by FOXP3 expression, suppress the activity of other immune cells and help prevent autoimmunity outside the thymus.

• A. Produces antibodies against bacterial toxins
• B. Drives expression of tissue-specific self-antigens in thymic cells ✓
• C. Encodes the T cell receptor
• D. Activates complement proteins in the blood

Why: AIRE (autoimmune regulator) promotes transcription of many tissue-restricted antigens in medullary thymic epithelial cells, allowing T cells reactive to those tissues to be deleted.

• A. Tuberculosis
• B. Type 1 diabetes ✓
• C. Influenza
• D. Malaria

Why: In type 1 diabetes, the immune system attacks insulin-producing beta cells in the pancreas. The other options are infectious diseases caused by pathogens.

• A. Hyperactivation leading to rapid proliferation
• B. Functional unresponsiveness after receiving an antigen signal without proper co-stimulation ✓
• C. Programmed self-destruction triggered by viral infection
• D. Migration to the thymus for reprogramming

Why: Anergy is a state of functional inactivation in which a lymphocyte fails to respond to antigen, typically because it received the antigen signal without the necessary co-stimulatory signal.

• A. To directly kill an active infection in the body
• B. To train the immune system to recognize a pathogen without causing the disease ✓
• C. To permanently replace damaged immune cells
• D. To suppress inflammation during an infection

Why: Vaccines expose the immune system to harmless components or forms of a pathogen, such as antigens or weakened organisms, so that it can build a response and memory without the person developing the full disease.

• A. Neutrophils
• B. Memory B cells and memory T cells ✓
• C. Mast cells
• D. Platelets

Why: After an initial exposure, some activated B and T lymphocytes differentiate into memory cells that persist for years or decades. On re-exposure to the same antigen, they mount a faster and stronger response.

• A. Active immunity is temporary and passive immunity is permanent
• B. Active immunity is produced by the body's own response, while passive immunity is received as ready-made antibodies from another source ✓
• C. Active immunity only occurs in children and passive immunity only in adults
• D. Active immunity involves T cells only and passive immunity involves B cells only

Why: In active immunity the body generates its own antibodies and memory cells, as with infection or vaccination. In passive immunity antibodies are transferred from another individual, such as maternal antibodies passed to a newborn or antibody injections, which offer protection that usually fades over weeks to months.

• A. Immunity acquired only by animals living in groups
• B. Protection of a population when a large enough proportion of people are immune, reducing spread to those who are not ✓
• C. A type of antibody produced in the herd of cells in the bone marrow
• D. The immune response of the liver to bacterial toxins

Why: When enough individuals in a community are immune, whether through vaccination or prior infection, the pathogen has difficulty spreading, which indirectly protects people who cannot be vaccinated or who did not respond to the vaccine.

• A. The original vaccine becomes toxic over time
• B. Immunological memory can wane over time, and a booster re-stimulates the immune system to strengthen memory ✓
• C. Boosters replace the white blood cells destroyed by the vaccine
• D. Boosters are needed only to change the vaccine's taste

Why: Antibody levels and memory cell numbers can decline over the years. A booster dose re-exposes the immune system to the antigen, prompting a rapid secondary response that restores and often enhances protective immunity.