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The Immune System

Chapter 8: The Immune System

3 min read Updated Mar 26, 2026
Read the entire chapter on one page Every section in order, with the sidebar tracking where you are as you scroll.
🎯 Diagnostic: Test Your Starting Level 24 questions (~2 per section). No prior reading required, see what you already know.

Aim to answer every question before checking. Missed questions point you to the sections you need most.

1. (8.1) The innate immune response is best described as:
A. Innate immunity responds in minutes to hours using germline-encoded pattern recognition receptors (TLRs). It does not remember the pathogen the next time around.
2. (8.1) Physical and chemical barriers of the innate immune system include:
D. These "first line" barriers keep most pathogens out before any cellular response is needed. Complement is innate but is a soluble system, not a physical barrier.
3. (8.2) Neutrophils are characterized by:
C. Neutrophils are the most abundant leukocyte and the first to arrive at infection sites. They phagocytose bacteria and release neutrophil extracellular traps (NETs).
4. (8.2) Macrophages and dendritic cells serve as:
B. Both phagocytose pathogens and present processed peptides on MHC II to naive CD4 T cells, activating adaptive immunity.
5. (8.3) Cell-mediated immunity is carried out primarily by:
D. Humoral immunity is antibody-based (B cells). Cell-mediated immunity is T-cell-based, important against intracellular pathogens and tumors.
6. (8.3) The defining features that distinguish adaptive from innate immunity are:
A. Adaptive immunity uses V(D)J recombination to generate billions of unique receptors and retains memory cells that respond faster on re-exposure.
7. (8.4) B-cell receptors recognize:
C. B-cell receptors (membrane-bound antibodies) bind antigen directly, recognizing 3D epitopes. T-cell receptors, in contrast, require MHC-presented peptides.
8. (8.4) Plasma cells are differentiated B cells that:
B. After activation, B cells either become memory cells or plasma cells (antibody factories with extensive rough ER).
9. (8.5) The most abundant antibody isotype in plasma is:
A. IgG is about 75% of plasma antibody and the only isotype that crosses the placenta. IgA dominates mucosal secretions; IgM is first in the primary response.
10. (8.5) IgE antibodies are responsible for:
D. IgE binds FcεRI on mast cells and basophils. Antigen cross-linking of bound IgE triggers release of histamine and other mediators.
11. (8.6) Cytotoxic T lymphocytes (CD8) recognize antigens presented on:
B. CD8 T cells police the cytosol by surveying MHC I peptides; if the cell presents viral or tumor peptides, the CTL kills it with perforin/granzymes. Rule: "8 × 1 = 8, 4 × 2 = 8" links CD8→MHC I and CD4→MHC II.
12. (8.6) Helper T lymphocytes (CD4) recognize antigens presented on:
C. Professional APCs (dendritic cells, macrophages, B cells) load extracellular peptides onto MHC II and present them to CD4 helpers.
13. (8.7) MHC class I molecules:
A. MHC I shows the outside world what is being made inside every cell. Viruses and tumor antigens are revealed this way, then CD8 T cells respond.
14. (8.7) Dendritic cells express MHC II and serve to:
D. Dendritic cells sample peripheral tissues, migrate to lymph nodes, and activate naive T cells. They are the most potent activators of a first-time adaptive response.
15. (8.8) Clonal selection refers to:
B. Each lymphocyte has one specificity. Antigen binding selects that clone to proliferate, producing effector and memory cells of the matched specificity.
16. (8.8) Memory B and T cells enable:
C. Memory cells persist for decades. On re-exposure they rapidly expand and produce high-affinity IgG, making vaccines effective.
17. (8.9) Active immunity involves:
D. Active immunity: host body makes the response and memory. Passive immunity: host receives someone else's antibodies, no memory formed.
18. (8.9) Passive immunity is exemplified by:
A. Passive immunity protects the newborn while its own immune system matures. It fades within months since no memory is generated.
19. (8.10) Autoimmune diseases result from:
B. Examples include type 1 diabetes (β cells), multiple sclerosis (myelin), and rheumatoid arthritis (joint tissue).
20. (8.10) Type I hypersensitivity (e.g., anaphylaxis, allergies) is mediated by:
C. "ACID" hypersensitivity mnemonic: I = Anaphylactic (IgE), II = Cytotoxic (IgG/IgM), III = Immune complex, IV = Delayed (T cell).
21. (8.2) Natural killer (NK) cells:
A. NK cells are innate lymphoid cells. They see "missing self" (loss of MHC I) and use perforin/granzymes to kill infected or malignant cells.
22. (8.5) IgM is most characteristic of:
D. IgM is the first isotype made by naive B cells; it circulates as a pentamer and is a potent complement activator. Class switching later produces IgG, IgA, or IgE.
23. (8.4) Class switching in B cells:
B. Activation-induced cytidine deaminase (AID) rearranges the heavy-chain constant region, swapping IgM for IgG, IgA, or IgE depending on cytokine context.
24. (8.10) Severe combined immunodeficiency (SCID) is characterized by:
C. SCID patients lack functional adaptive immunity and cannot survive common infections. Gene-therapy and bone-marrow transplant can reconstitute the missing lineage.

You get a paper cut. Within seconds, the area turns red and starts to swell. Within hours, white blood cells have flooded the wound, killing bacteria before they can establish an infection. Within days, if any pathogen managed to slip through, your body has manufactured custom-built molecular weapons - antibodies - designed to neutralize that specific invader. And if the same pathogen ever shows up again, even years later, your immune system remembers it and destroys it before you ever feel a symptom.

This is not one system. It is two systems working in concert: a fast, general-purpose defense force (innate immunity) and a slow, precision-guided strike team (adaptive immunity). Understanding how these two arms coordinate is the key to nearly every immune system question on the MCAT.

The Castle and the Intelligence Agency

Think of your body as a medieval kingdom under constant threat of invasion. Your immune system has two layers of defense.

Innate immunity is the castle. The skin is the outer wall. Mucus and stomach acid are the moat. If invaders breach the wall, guards are already stationed inside - neutrophils, macrophages, and NK cells - who attack anything that does not belong. They do not need to identify the specific enemy. They just recognize “not self” and destroy it. This response is immediate but generic.

Adaptive immunity is the intelligence agency. After the castle guards encounter a new enemy, they send a detailed description (antigen presentation) to headquarters (lymph nodes). The intelligence agency then manufactures precision weapons: B cells produce antibodies that tag that exact invader, while T cells become assassins trained to kill cells infected by that exact pathogen. Most importantly, the agency keeps the enemy’s file on record forever (memory cells) - so the next encounter triggers an instant, devastating response.

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