Which cell is responsible for immunity
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Here is a complete, textbook-grounded answer:
Cells Responsible for Immunity
The lymphocyte is the principal cell responsible for immunity. Lymphocytes are a heterogeneous population with three major functional types, each serving a distinct role.
1. T Lymphocytes (T Cells) - Cell-Mediated Immunity
T cells are responsible for cell-mediated immunity. They are characterized by the surface markers CD2, CD3, CD5, CD7, and T-cell receptors (TCRs). They have a long lifespan and handle:
- Delayed hypersensitivity reactions
- Graft rejection and graft-versus-host reactions
- Defense against intracellular organisms (e.g., Mycobacterium tuberculosis, Brucella)
- Defense against tumor cells
- Producing cytokines: IL-2 (T cell proliferation), IL-3, IL-4, IL-5, IL-6, lymphotoxin, and migratory inhibitory factor
T cells make up the majority (~80-90%) of circulating lymphocytes and have lifespans of months to years.
2. B Lymphocytes (B Cells) - Humoral (Antibody-Mediated) Immunity
B cells are characterized by CD9, CD19, CD20 markers and surface IgM/IgD. They are responsible for humoral immunity by:
- Recognizing antigens
- Transforming into plasma cells, which produce large quantities of antibodies (immunoglobulins)
- Generating memory B cells with a long lifespan
B cells make up only 10-20% of circulating lymphocytes.
3. Natural Killer (NK) Cells - Innate Cytotoxic Immunity
NK cells are non-T, non-B lymphocytes expressing CD16a, CD56, and CD94. They:
- Are not antigen-specific - no T-cell receptor, no immunoglobulin
- Destroy virus-infected cells and some tumor cells via a cytotoxic mechanism
- Act as a rapid first-line defense (part of innate immunity)
4. Other Key Immune Cells (Innate Immunity)
Beyond lymphocytes, these cells also contribute to immunity:
| Cell | Role |
|---|---|
| Neutrophils | First responders; phagocytose bacteria |
| Macrophages | Phagocytosis, antigen presentation, cytokine secretion |
| Dendritic cells | Antigen presentation to T cells (link innate to adaptive) |
| Eosinophils | Defense against parasites, allergic reactions |
| Mast cells/Basophils | Allergic and inflammatory responses |
| Plasma cells | Derived from B cells; antibody factories |
Summary
| Cell | Immunity Type | Key Function |
|---|---|---|
| T cells | Adaptive - Cellular | Kill infected/tumor cells, regulate immune response |
| B cells / Plasma cells | Adaptive - Humoral | Produce antibodies |
| NK cells | Innate - Cytotoxic | Kill virus-infected and tumor cells without prior sensitization |
| Neutrophils/Macrophages | Innate | Phagocytosis, inflammation |
The figure below shows a large granular lymphocyte (NK/T cell type) as seen on peripheral blood smear:
Large granular lymphocyte with azurophilic cytoplasmic granules (Wright-Giemsa, 1000x)
Sources:
- Histology: A Text and Atlas with Correlated Cell and Molecular Biology, p. 529-530
- Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 2417-2426
Antigen and antibody
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Here is a complete, textbook-grounded explanation of antigens and antibodies:
Antigen and Antibody
ANTIGEN
Definition
"An antigen is any substance that can induce a specific immune response."
- Histology: A Text and Atlas, p. 1153
More specifically: an antigen is usually a large, structurally complex foreign molecule - such as a protein, polysaccharide, or glycolipid - that is recognized by cells of the immune system. It does not have to be pathogenic in origin; it simply needs to be "recognized as foreign."
Antigen vs. Immunogen vs. Hapten
| Term | Meaning |
|---|---|
| Antigen | Any substance recognized by the immune system |
| Immunogen | An antigen that can actually trigger an immune response (all immunogens are antigens, but not vice versa) |
| Hapten | A small molecule (usually <20 kDa) that cannot trigger immunity alone but can if bound to a larger carrier protein (e.g., penicillin) |
Epitopes (Antigenic Determinants)
Each antigen has multiple epitopes - specific regions/sites that are recognized by the immune system. A single antigen can contain many different epitopes, and copies of the same epitope make it multivalent, greatly increasing antibody binding strength.
Types of Antigens
- T-dependent antigens (e.g., proteins): require T-cell help to stimulate B cells; produce high-affinity antibody + memory cells
- T-independent antigens (e.g., polysaccharides): stimulate B cells directly without T cells; produce lower-affinity antibody, no memory cells - reason why polysaccharide vaccines don't work in children under 2 years
ANTIBODY (Immunoglobulin)
Definition
Antibodies are glycoproteins produced by plasma cells (differentiated B cells) that specifically bind to the antigen that triggered their production. They belong to the immunoglobulin (Ig) family.
Structure of an Antibody (IgG model)
The basic unit is a Y-shaped tetramer consisting of:
- 2 heavy (H) chains - define the antibody class
- 2 light (L) chains - either kappa (κ) or lambda (λ) type
- Chains are connected by disulfide bonds
Two Functional Regions:
| Region | Fragment | Function |
|---|---|---|
| Variable region (VH/VL) | Fab - antigen-binding fragment | Binds to the specific antigen (2 identical sites per IgG) |
| Constant region (CH) | Fc - crystallizable fragment | Binds complement, interacts with macrophages/NK cells/mast cells, controls transport across placenta/mucosa |
Papain digestion yields:
- 2 Fab fragments - each with 1 antigen-binding site
- 1 Fc fragment - effector functions
The 5 Classes of Antibodies (Isotypes)
Each class is defined by its heavy chain type:
| Class | Heavy Chain | Structure | % in Blood | Key Functions |
|---|---|---|---|---|
| IgG | γ (gamma) | Monomer | 75-85% | Most abundant; neutralizes antigens; activates phagocytosis; crosses the placenta (passive immunity to fetus) |
| IgM | μ (mu) | Pentamer | 5-10% | First antibody produced in any immune response; most effective at activating complement |
| IgA | α (alpha) | Dimer (secretory) | 10-15% | Protects mucosae; found in saliva, milk, tears, mucus; secretory immunity |
| IgD | δ (delta) | Monomer | 0.001% | Acts as antigen receptor on B cell surface; triggers initial B cell activation |
| IgE | ε (epsilon) | Monomer | 0.002% | Bound to mast cells/basophils; triggers allergic reactions; defense against parasitic worms |
Antigen-Antibody Interaction
The binding between antigen and antibody is reversible and non-covalent, involving:
- Hydrophobic bonds
- Hydrogen bonds
- Van der Waals forces
- Ionic interactions
These forces only work when the antigen fits closely into the complementary binding site of the antibody - like a lock and key.
Key concepts:
- Affinity - tightness of binding at a single site (IgG has 2 sites)
- Avidity - total binding strength of all sites together; IgM (10 sites as pentamer) has far greater avidity than IgG despite similar per-site affinity
- Affinity maturation - as the immune response progresses, antibody affinity increases over time
Antigen-Antibody: Summary Comparison
| Feature | Antigen | Antibody |
|---|---|---|
| Nature | Foreign substance (protein, polysaccharide, etc.) | Glycoprotein (immunoglobulin) |
| Produced by | External source / pathogen | Plasma cells (from B lymphocytes) |
| Role | Triggers immune response | Neutralizes / eliminates antigen |
| Specific region | Epitope (on antigen) | Paratope (combining site on antibody) |
| Size | Usually large (>10 kDa for immunogens) | ~150 kDa (IgG) to ~900 kDa (IgM pentamer) |
Sources:
- Histology: A Text and Atlas with Correlated Cell and Molecular Biology, p. 1153-1154
- Sherris & Ryan's Medical Microbiology, 8th Ed., p. 77-87
- Junqueira's Basic Histology, 17th Ed., p. 671-672
- Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1087
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