Adaptive immunity

What is adaptive immunity?

Adaptive immunity is your body’s specific and memory-based defense system.

• Learns to recognize specific pathogens (like viruses, bacteria, toxins)

• Takes time to activate (usually a few days after infection)

• Uses two main types of immune cells:
  

  • B cells – produce antibodies that lock onto invaders

  • T cells – help coordinate the response or directly kill infected cells

• Creates memory cells that stay in your body after the infection

• Responds faster and stronger if the same pathogen comes back

• Basis of how vaccines work — by teaching your immune system to recognize a threat before you ever get sick

How do different blood cells develop from stem cells in the bone marrow?

All blood cells start from a blood stem cell in the bone marrow. This stem cell can become two major types of stem cells:

1. Myeloid stem cell

   • Produces:

     • Red blood cells (carry oxygen)

     • Platelets (help blood clot)

     • Granulocytes (types of white blood cells: neutrophils, eosinophils, basophils)

     • Monocytes (develop into macrophages)

2. Lymphoid stem cell

   • Produces:

     • B lymphocytes (make antibodies)

     • T lymphocytes (help kill infected or abnormal cells)

     • Natural killer (NK) cells (kill infected or cancerous cells without needing antibodies)

What are the main roles of B cells in adaptive immunity?

B cells are a special type of immune cell that help you fight off specific germs — and remember them for next time.

• First, each B cell has tiny sensors on its surface. These sensors can recognize one specific invader — like a virus or bacteria.

• When a B cell detects its match (called an antigen), it gets activated. Sometimes it also gets help from another immune cell called a helper T cell.

• Once activated, the B cell transforms into something called a plasma cell. Plasma cells are like factories — they pump out antibodies.

• Antibodies are proteins that float around your body, stick to invaders, and help destroy them. They either block the germ or mark it so other immune cells can attack.

• After the infection is gone, some B cells become memory B cells. These stay in your body and help you respond faster if the same germ tries to attack again.

What is an antibody, and how does it help kill or remove pathogens?

• An antibody is a Y-shaped protein made by B cells that specifically binds to antigens on pathogens.

• Antibodies don’t kill pathogens directly, but they help eliminate them through several key mechanisms:

  • Neutralization: Blocks pathogens from entering or damaging cells.

  • Opsonization: Tags pathogens so phagocytes (like macrophages) can engulf and destroy them.

  • Complement activation: Triggers a protein cascade that punches holes in pathogen membranes.

  • Agglutination: Clumps pathogens together to make them easier to eliminate.

Why does the adaptive immune response take longer to activate than the innate response?

• The adaptive immune system is highly specific and must first identify the exact antigen on the pathogen.

• It takes time to find the few B and T cells that have receptors matching that specific antigen.

• Once a match is found, those cells must be activated (often with help from other immune cells).

• The matched cells then go through clonal expansion, multiplying rapidly to build a strong response.

• Only after this buildup can they produce antibodies or kill infected cells.

• This whole process takes several days, which is why adaptive immunity is slower to start than the fast, pre-programmed innate response.

What are memory cells and how do they improve immune responses?

• Memory cells: Long-lived B and T cells generated after initial pathogen exposure.

• Allow rapid, powerful, specific response upon subsequent exposure to the same pathogen.

• Provide lasting immunity—second exposure response is faster and stronger.

How do central and peripheral tolerance prevent autoimmunity?

Central Tolerance

• Happens in primary lymphoid organs:

  • Bone marrow (for B-cells)

  • Thymus (for T-cells)

• During early development, immature lymphocytes are tested:

  • If they recognize self-antigens too strongly, they are eliminated or made inactive

• This prevents self-reactive cells from entering the bloodstream

Peripheral Tolerance

• Happens after lymphocytes enter circulation

• Some self-reactive cells escape central tolerance

• Peripheral tolerance uses mechanisms to suppress or shut down these cells:

  • Regulatory T-cells

  • Anergy (inactivation)

  • Apoptosis (cell death)

What can lead to the development of autoimmune diseases?

Autoimmune diseases occur when the immune system mistakenly attacks the body’s own healthy cells, treating them as foreign

Causes of Autoimmunity:

1. Genetic factors – inherited risk (e.g., HLA gene variants)

2. Infections – especially viruses, which may trigger cross-reactions

3. Hormonal influences – e.g., estrogen may increase risk (many autoimmune diseases are more common in females)

4. Chronic inflammation – can cause exposure of hidden self-antigens

5. Failure of tolerance mechanisms (central or peripheral)

How do helper T cells and cytotoxic T cells function differently?

Feature	Helper T-Cells	Cytotoxic T-Cells
Recognize antigens on	MHC Class II (from APCs)	MHC Class I (on all body cells)
Main role	Coordinate the immune response	Directly kill infected cells
Function	Activate B-cells, macrophages, and cytotoxic T-cells	Use enzymes to kill infected or cancerous cells

How is the adaptive immune system involved in cancer surveillance and treatment?

The adaptive immune system helps detect and destroy cancer cells by recognizing abnormal antigens on their surface.

Key Players

• Cytotoxic T-cells (CD8⁺):

  • Can recognize cancer cells displaying abnormal peptides on MHC I

  • They kill these cells before the tumor can grow

• Helper T-cells (CD4⁺):

  • Release cytokines to recruit and activate cytotoxic T-cells and macrophages

• B-cells/Antibodies:

  • Can bind to cancer cell markers and tag them for destruction

In treatment:

• Some immunotherapies (like CAR-T cell therapy) take a patient’s T-cells and genetically engineer them to target cancer cells

How does vaccination work?

Vaccination works by training your adaptive immune system to recognize and fight a specific pathogen before you ever get infected.

• A vaccine contains weakened, killed, or harmless parts of a pathogen — like proteins or genetic material.

• Your immune system treats the vaccine as a real threat and activates B and T cells to fight it.

• B cells produce antibodies, and memory cells (both B and T) are formed.

• These memory cells stay in your body and “remember” the pathogen.

• If you’re exposed to the real infection later, your immune system responds faster and stronger, often stopping the illness before it causes symptoms.