Free MCAT study guide — Biological and Biochemical Foundations of Living Systems
The immune system is heavily tested on the MCAT across multiple content areas. You must understand the distinction between innate (nonspecific) and adaptive (specific) immunity, including the cells, molecules, and mechanisms involved in each. The innate immune system provides immediate defense through physical barriers (skin, mucous membranes), chemical barriers (lysozyme, stomach acid, defensins), cellular components (neutrophils, macrophages, dendritic cells, natural killer cells), and the complement system. The adaptive immune system provides specific, long-lasting immunity through T cells (cell-mediated immunity) and B cells (humoral immunity). You need to know the processes of antigen presentation via MHC I and MHC II, T cell activation and differentiation, B cell activation and antibody production, and the generation of immunological memory. The MCAT frequently tests concepts like clonal selection, the difference between active and passive immunity, hypersensitivity reactions, autoimmunity, and immunodeficiency. Passage-based questions may present experimental data from ELISA assays, flow cytometry, or vaccine trials.
The innate immune system provides immediate, nonspecific defense against pathogens. Physical barriers include the skin (keratinized epithelium), mucous membranes, cilia in the respiratory tract, and the acidic pH of the stomach and vagina. Chemical defenses include lysozyme (in tears and saliva, degrades peptidoglycan in bacterial cell walls), defensins (antimicrobial peptides), and the acidic environment of the stomach (pH 1-2). When pathogens breach these barriers, pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) on macrophages and dendritic cells recognize pathogen-associated molecular patterns (PAMPs) like lipopolysaccharide (LPS) and flagellin. This triggers phagocytosis and the release of pro-inflammatory cytokines (IL-1, IL-6, TNF-alpha) that initiate the inflammatory response. The cardinal signs of inflammation are redness (rubor), heat (calor), swelling (tumor), and pain (dolor), caused by vasodilation, increased vascular permeability, and recruitment of immune cells.
Key innate immune cells include neutrophils (most abundant white blood cells, first responders, phagocytic), macrophages (derived from monocytes, phagocytic, present antigens to T cells), dendritic cells (most important antigen-presenting cells, bridge innate and adaptive immunity), natural killer (NK) cells (kill virus-infected cells and tumor cells by recognizing the absence of MHC I), mast cells (release histamine during allergic responses), eosinophils (combat parasitic infections), and basophils (release histamine and heparin). The complement system consists of over 30 plasma proteins that are activated in a cascade. The three activation pathways (classical, lectin, and alternative) all converge on C3 convertase, which cleaves C3 into C3a (anaphylatoxin, promotes inflammation) and C3b (opsonin, marks pathogens for phagocytosis). The terminal pathway forms the membrane attack complex (MAC, C5b-C9), which creates pores in pathogen membranes, causing lysis.
T cells mature in the thymus and are responsible for cell-mediated immunity. T cell receptors (TCRs) recognize antigen fragments presented on MHC molecules. CD8+ cytotoxic T cells recognize antigens presented on MHC class I molecules (found on all nucleated cells) and kill infected or cancerous cells by releasing perforin (creates pores) and granzymes (induce apoptosis). CD4+ helper T cells recognize antigens on MHC class II molecules (found only on antigen-presenting cells: macrophages, dendritic cells, B cells). Helper T cells differentiate into subsets: Th1 cells activate macrophages and CD8+ T cells (cell-mediated immunity), Th2 cells activate B cells and promote antibody production (humoral immunity), Th17 cells recruit neutrophils, and regulatory T cells (Tregs) suppress immune responses to prevent autoimmunity. T cell activation requires two signals: TCR-MHC interaction (signal 1) and co-stimulatory molecules like B7-CD28 (signal 2). Without signal 2, T cells become anergic (unresponsive).
B cells mature in the bone marrow and are responsible for humoral (antibody-mediated) immunity. Each B cell expresses a unique B cell receptor (membrane-bound antibody) generated through V(D)J recombination, which creates enormous antibody diversity. Upon encountering antigen, B cells internalize it, process it, and present fragments on MHC II to helper T cells (T-dependent activation). The helper T cell provides co-stimulation and cytokines, causing the B cell to proliferate (clonal expansion) and differentiate into plasma cells (secrete antibodies) and memory B cells (provide long-lasting immunity). Antibody classes include IgM (first produced, pentamer, activates complement), IgG (most abundant in blood, crosses placenta, opsonizes), IgA (found in secretions -- saliva, breast milk, mucus), IgE (binds mast cells, mediates allergic reactions and anti-parasitic defense), and IgD (B cell surface receptor, function in signaling). Class switching changes the antibody constant region but not antigen specificity. Affinity maturation through somatic hypermutation increases antibody binding strength during an immune response.
The primary immune response occurs upon first exposure to an antigen and takes 7-10 days to produce significant antibody levels. IgM is produced first, followed by class switching to IgG. The secondary immune response upon re-exposure is faster (1-3 days), stronger (higher antibody titers), longer-lasting, and predominantly IgG, due to the activation of memory B and T cells. This is the basis of vaccination. Active immunity involves stimulating the immune system to produce its own antibodies and memory cells -- through natural infection or vaccination (attenuated, inactivated, subunit, mRNA, or toxoid vaccines). Passive immunity involves transferring pre-formed antibodies -- naturally through maternal IgG crossing the placenta or IgA in breast milk, or artificially through injection of immunoglobulin (e.g., anti-venom, anti-Rh antibodies). Active immunity is long-lasting; passive immunity is temporary.
Hypersensitivity reactions represent overactive immune responses. Type I (immediate/anaphylactic) involves IgE-mediated mast cell degranulation, causing histamine release -- examples include allergies, asthma, and anaphylaxis. Type II (cytotoxic) involves IgG or IgM antibodies targeting cell surface antigens -- examples include hemolytic disease of the newborn and transfusion reactions. Type III (immune complex) involves antigen-antibody complexes depositing in tissues, causing inflammation -- examples include lupus (SLE) and serum sickness. Type IV (delayed-type) is T cell-mediated and takes 24-72 hours -- examples include contact dermatitis, tuberculin skin test, and transplant rejection. Autoimmune diseases occur when the immune system attacks self-antigens due to failure of central tolerance (deletion of self-reactive lymphocytes in thymus/bone marrow) or peripheral tolerance (regulatory T cells, anergy). Examples include type 1 diabetes (T cells destroy beta cells), rheumatoid arthritis, multiple sclerosis, and Graves disease.
MHC I is on all nucleated cells and presents endogenous (intracellular) antigens to CD8+ T cells. MHC II is on APCs only and presents exogenous antigens to CD4+ T cells.
The rule of 8: CD8 x MHC I = 8, CD4 x MHC II = 8.
NK cells kill cells LACKING MHC I ('missing self' hypothesis).
IgG is the only antibody that crosses the placenta.
IgA is the most abundant antibody overall (found in secretions).
IgE mediates Type I hypersensitivity (allergic) reactions by binding Fc receptors on mast cells.
Clonal selection: antigen selects the B/T cell with the matching receptor for expansion.
V(D)J recombination generates antibody diversity before antigen exposure.
Somatic hypermutation and affinity maturation increase antibody affinity during an immune response.
Complement C3b is an opsonin; MAC (C5b-C9) lyses cells directly.
Neutrophils are first responders; macrophages and dendritic cells are key APCs.
Active immunity is long-lasting (memory cells); passive immunity is temporary (pre-formed antibodies).
Fever is caused by pyrogens (IL-1, TNF-alpha) acting on the hypothalamus to raise the temperature set point.
Interferons (IFN-alpha, IFN-beta) are released by virus-infected cells to warn neighboring cells.
Confusing MHC I (all nucleated cells, CD8+ T cells) with MHC II (APCs only, CD4+ T cells).
Forgetting that red blood cells and platelets LACK MHC I because they are anucleate.
Mixing up active immunity (body makes its own antibodies) with passive immunity (receives pre-formed antibodies).
Confusing the four types of hypersensitivity -- remember Type I is immediate/IgE, Type IV is delayed/T cell-mediated.
Thinking B cells only need antigen to activate -- T-dependent activation also requires helper T cell co-stimulation.
Assuming antibody diversity comes only from gene recombination -- somatic hypermutation, junctional diversity, and combinatorial joining all contribute.
Forgetting that class switching changes antibody effector function (constant region) but not antigen specificity (variable region).
Create a comparison chart of innate vs. adaptive immunity listing the cells, speed, specificity, and memory for each. Then create sub-charts comparing CD4+ vs. CD8+ T cells and the five antibody classes. The MCAT loves questions that require you to identify which cell type or molecule is responsible for a specific immune function, so being able to quickly recall these distinctions is essential.
Practice interpreting immunology experiments: ELISA results showing antibody titers over time (primary vs. secondary response), flow cytometry data identifying cell populations by surface markers (CD4, CD8, CD19 for B cells), and vaccination scenarios. Be comfortable predicting what would happen in immunodeficiency states -- for example, what infections would increase in a patient lacking CD4+ T cells (as in HIV/AIDS) versus one lacking B cells.
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