Principles Of Immune Response

Innate Versus Adaptive Immunity

Innate immunity is the body's first line of defense: present at birth, immediate (minutes to hours), non-specific, and without memory. Its players are physical barriers (skin, mucosa), phagocytes (neutrophils, macrophages), natural killer (NK) cells, the complement system, and soluble factors such as lysozyme and acute-phase proteins (C-reactive protein, CRP). Adaptive immunity is slower (days), antigen-specific, and generates immunologic memory. It has two arms: humoral (B cells producing antibody) and cell-mediated (T cells).

The MLS exam loves the discriminator that only adaptive immunity shows memory and specificity. NK cells, although lymphocytes by lineage, are functionally innate because they kill without prior sensitization and without antigen-specific receptors.

Antibody (Immunoglobulin) Structure And Classes

Every immunoglobulin monomer is a Y-shaped molecule of two heavy chains and two light chains (kappa or lambda) joined by disulfide bonds. The Fab region binds antigen; the Fc region mediates effector functions (complement fixation, placental transport, receptor binding). The heavy-chain class defines the isotype.

A classic trap: a newborn with a positive IgM antibody to an organism indicates true neonatal (congenital) infection, because IgM does not cross the placenta. Maternal IgG, in contrast, transfers across the placenta and can give a falsely "positive" IgG in the infant without active infection.

Primary Versus Secondary (Anamnestic) Response

The kinetics of the antibody response are heavily tested because they explain how serology timing is interpreted.

• Primary response (first exposure): a lag phase of several days, then IgM rises first, peaking at roughly 7-10 days, with a low overall titer and low antibody avidity (binding strength). IgG appears later and at lower levels.
• Secondary / anamnestic response (re-exposure): memory B cells trigger a faster, stronger, and longer response dominated by IgG, with higher titer and higher avidity.

Worked example: A patient's acute serum is IgM-positive and IgG-negative for a virus; two weeks later the convalescent serum shows a four-fold rise in IgG. The IgM-then-IgG pattern with a rising titer confirms a recent, acute primary infection. A fourfold (two-tube-dilution) rise between acute and convalescent specimens is the standard serologic criterion for acute infection.

Cells And Mediators

T cells mature in the thymus and bear CD3. CD4+ helper T cells coordinate the response and are the target of HIV; CD8+ cytotoxic T cells kill virus-infected and tumor cells. B cells mature into antibody-secreting plasma cells and memory cells. The normal CD4:CD8 ratio is approximately 2:1; an inverted ratio (<1) is a hallmark of advanced HIV disease.

Complement can be activated by three pathways: the classical pathway (triggered by IgM or IgG immune complexes, starting at C1), the alternative pathway (microbial surfaces, no antibody), and the lectin pathway (mannose-binding lectin). All three converge on C3, the central component, and proceed to the C5-C9 membrane attack complex (MAC) that lyses cells.

High-Yield Traps

• IgM is the largest (pentamer) and the best complement fixer, not IgG, despite IgG being most abundant.
• Only IgG crosses the placenta; IgM in a newborn signals congenital infection.
• Affinity is the strength of a single bond; avidity is the total binding of a multivalent antibody (IgM has high avidity from 10 binding sites despite low affinity per site).
• A positive antibody result reflects exposure/immunity, not necessarily active disease; pair it with IgM, titer trends, or antigen testing.

Antigens, Haptens, And The Basis Of Specificity

An antigen is any substance that the immune system recognizes; an immunogen is an antigen that can provoke a response on its own. Larger, more complex, foreign molecules (proteins, polysaccharides) are more immunogenic than small or self-like molecules. The precise part of an antigen that antibody binds is the epitope (antigenic determinant). A hapten is a small molecule that cannot stimulate antibody production alone but becomes immunogenic when coupled to a carrier protein; penicillin acting as a hapten on red cells is the classic cause of drug-induced hemolytic anemia.

This is why the exam pairs the word hapten with carrier-dependent immune responses.

Specificity arises because each B-cell clone, through gene rearrangement of variable regions, produces antibody to a single epitope. Clonal selection then expands the clone whose receptor matches the antigen, and class switching lets that clone change its heavy-chain isotype (IgM to IgG, IgA, or IgE) while keeping the same antigen specificity. Memory cells from this process are what make the secondary response faster and stronger.

Active Versus Passive Immunity

Understanding how immunity is acquired is frequently tested and underlies vaccine and immunoglobulin questions.

Active immunity is the host's own response and creates lasting memory; passive immunity is borrowed antibody that protects immediately but fades and confers no memory. RhIG (Rh immune globulin) given to an Rh-negative mother is a passive-artificial example used to prevent hemolytic disease of the newborn. A common trap is calling vaccine-induced immunity passive — it is active because the host produces the antibody.