Immunology Essentials

Immunology on Step 1 rewards a clean mental model over rote lists. If you can trace an antigen from the moment it breaches a barrier through innate recognition, antigen presentation, and the adaptive response, most questions resolve to a single logical answer. This lesson builds that spine: the two arms of immunity, how MHC directs T cells, what antibodies and complement actually do, and how the four hypersensitivities and immunodeficiencies fit on top.

Core Idea

• Innate immunity is fast, fixed, and non-specific (barriers, neutrophils, macrophages, NK cells, complement); adaptive immunity is slow on first exposure, specific, and remembers (T and B lymphocytes).
• MHC is the presentation system that tells T cells what to do: MHC I is on all nucleated cells and engages CD8 T cells; MHC II is on professional antigen-presenting cells and engages CD4 T cells.
• CD4 helper T cells are the coordinators of nearly the entire adaptive response, which is why their loss (HIV) collapses immunity broadly.

Innate vs. Adaptive Immunity

Innate immunity is the first responder. It uses physical barriers (skin, mucus), pattern-recognition receptors that detect conserved microbial motifs (PAMPs), and cells that act within minutes to hours: neutrophils (first to a bacterial site), macrophages (phagocytose and present antigen), dendritic cells (the key link to adaptive immunity), and natural killer (NK) cells. It has no memory — the second exposure looks like the first.

Adaptive immunity is antigen-specific and clonal. On first exposure it is slow (days) because rare specific clones must expand, but it generates memory, so re-exposure produces a faster, stronger response. It has two arms: cell-mediated (T cells) and humoral (B cells and antibodies).

Cells and Their Roles

• Neutrophils — acute bacterial killing via phagocytosis; dominate pus.
• Macrophages — phagocytosis, antigen presentation on MHC II, cytokine release; activated by CD4 Th1 cells and interferon-gamma.
• Dendritic cells — the most important antigen-presenting cell for activating naive T cells.
• NK cells — kill virus-infected and tumor cells that downregulate MHC I ("missing self") and perform antibody-dependent cellular cytotoxicity.
• B cells — become plasma cells that secrete antibody; also present antigen.
• T cells — CD4 helpers and CD8 cytotoxic effectors.

MHC and T Cell Activation

MHC class I is expressed on all nucleated cells, presents endogenous (cytosolic/viral) peptides, and is recognized by CD8 cytotoxic T cells. A handy rule: MHC I x CD8 = 8, and MHC II x CD4 = 8.

MHC class II is expressed only on professional antigen-presenting cells (dendritic cells, macrophages, B cells), presents exogenous (phagocytosed) peptides, and is recognized by CD4 helper T cells.

Once activated, CD4 T cells differentiate — Th1 (activate macrophages and CD8 cells), Th2 (help B cells and drive IgE/eosinophils), Th17, and others — and provide the help B cells need for class switching. CD8 T cells directly kill infected cells via perforin and granzyme. B cells recognize native antigen through their receptor, receive CD4 help, and mature into antibody-secreting plasma cells and memory cells.

Antibodies and Complement

Antibody isotypes: IgM is the first made and forms a pentamer (best at complement fixation, marks acute infection); IgG is the most abundant, mediates opsonization, neutralization, and complement fixation, and is the only isotype that crosses the placenta; IgA is a dimer guarding mucosal surfaces and breast milk; IgE binds mast cells to drive type I hypersensitivity and antiparasitic defense; IgD sits on naive B cells with unclear function.

The complement system is a cascade of plasma proteins with three activation paths (classical, triggered by antigen-antibody complexes; alternative; and lectin). Its key jobs: opsonization (C3b), anaphylaxis/chemotaxis (C3a, C5a), and the membrane attack complex (C5b-9) that lyses cells — especially important against encapsulated organisms and Neisseria.

Hypersensitivity, Immunity, and Immunodeficiency

The four hypersensitivities (ACID): Type I — Anaphylactic/allergic, IgE- and mast-cell-mediated (allergy, anaphylaxis, atopy); Type II — Cytotoxic, antibody against cell-surface antigens (autoimmune hemolytic anemia, Goodpasture, hemolytic disease of the newborn); Type III — Immune complex, antigen-antibody deposits (serum sickness, SLE, post-streptococcal glomerulonephritis); Type IV — Delayed, T-cell-mediated with no antibody (PPD/TB test, contact dermatitis, transplant rejection).

Active immunity is your own response to infection or vaccine (slow onset, long-lasting, memory). Passive immunity is preformed antibody given directly (rapid, temporary, no memory) — e.g., maternal IgG or antitoxin. Immunodeficiencies are grouped as B-cell/antibody defects (recurrent encapsulated bacterial infections), T-cell defects (viral, fungal, opportunistic infections), combined, phagocyte, and complement defects.

High-Yield Exam Patterns

• "Which cell presents to CD8?" Any nucleated cell via MHC I; CD4 requires MHC II on a professional APC.
• Tumor or virus-infected cell with LOW MHC I killed without prior sensitization = NK cell.
• Only IgG crosses the placenta; IgM cannot — cord blood IgM implies fetal infection.
• Match the hypersensitivity by mechanism: IgE = I, antibody-to-cell = II, immune complex = III, T-cell/delayed = IV.
• Recurrent Neisseria infections point to a terminal complement (C5-C9) deficiency.
• Preformed antibody given for immediate protection = passive immunity (fast, no memory).

Common Traps to Avoid

• Confusing Type II (antibody against cells) with Type III (soluble immune complexes) — the location of antigen distinguishes them.
• Assuming innate immunity has memory — it does not; only adaptive immunity remembers.
• Swapping the MHC-CD pairing; keep the "multiply to 8" rule straight.
• Calling maternal antibody "active" — it is passive and wanes over months.
• Forgetting that CD4 cells help CD8 and B cells alike, so CD4 loss cripples both arms.