Lecture 10 - Antigen presenting and major histocompatibility complex

Question 1

What is an antigen?

Answer 1

A substance that has the ability to provoke an immune response

Question 2

How do T cells see antigens ?

Answer 2

T cells only recognise Ag when presented in the context of self Major Histocompatibility complex (MHC)

Distinguishes B and T cell recognition

Question 3

What are APC (Antigen Presenting cells)

Answer 3

Different cell types

mediate the cellular immune response

Presents antigen to the cells that will deal with the antigens

Examples:
- Dendritic cells
- Macrophages
- B lymphocytes

Question 4

What is antigen presentation?

Answer 4

Antigen presentation is the process by which specialized immune cells, known as antigen-presenting cells (APCs), process and display peptide fragments from pathogens on their surface using MHC molecules. These displayed peptides are then recognized by T cells, initiating an adaptive immune response.

Question 5

Q2: How do T cells recognize antigens?

Answer 5

T cells recognize antigens only when they are presented on the surface of other cells in combination with self-MHC molecules. This contrasts with B cells, which can recognize free-floating antigens

Question 6

Q3: What is an antigen?

Answer 6

An antigen is any substance capable of inducing an immune response, particularly by being recognized as foreign by the immune system.

Question 7

Q4: What are the main types of antigen-presenting cells?

Answer 7

The main APCs are:

Dendritic cells – most efficient at activating naive T cells.

Macrophages – engulf and digest pathogens; activate memory T cells.

B lymphocytes – present antigens bound via their B cell receptors (BCRs).

Question 8

Q5: What is the role of APCs in immunity?

Answer 8

APCs ingest pathogens, process them, and display antigen fragments on their surface using MHC molecules. This presentation activates T cells, which then coordinate or execute immune responses.

Question 9

Q6: What is the Major Histocompatibility Complex (MHC)?

Answer 9

MHC is a group of genes that code for proteins found on the surface of cells that help the immune system recognize foreign substances. In humans, MHC is known as HLA (Human Leukocyte Antigen), located on chromosome 6.

Question 10

Q7: What is the main function of MHC molecules?

Answer 10

Their primary function is to present peptide antigens to T cells to trigger a specific adaptive immune response.

Question 11

Q8: What are the structural differences between MHC Class I and MHC Class II molecules?

Answer 11

MHC Class I: One alpha chain (α1, α2, α3) and one β2-microglobulin; binding groove is between α1 and α2.

MHC Class II: Two chains (α and β), each with two domains (α1, α2 and β1, β2); binding groove is between α1 and β1.

Question 12

Q9: Where are MHC Class I and Class II molecules expressed?

Answer 12

MHC Class I: Found on all nucleated cells.

MHC Class II: Found only on APCs like dendritic cells, macrophages, and B cells.

Question 13

Q10: What types of antigens do MHC Class I and Class II present?

Answer 13

MHC Class I: Presents endogenous (intracellular) antigens (e.g., viral proteins).

MHC Class II: Presents exogenous (extracellular) antigens (e.g., bacterial proteins).

Question 14

Q11: Which T cells recognize MHC Class I and Class II?

Answer 14

MHC Class I: Recognized by CD8+ cytotoxic T cells

MHC Class II: Recognized by CD4+ helper T ce

Question 15

Q12: How are antigens processed for MHC Class I presentation?

Answer 15

Intracellular antigens are degraded by proteasomes.

Peptides are transported into the endoplasmic reticulum (ER) via TAP (Transporter Associated with Antigen Processing).

Peptides are loaded onto MHC Class I molecules.

The MHC-peptide complex is trafficked to the cell surface.

Question 16

Q13: How are antigens processed for MHC Class II presentation?

Answer 16

Exogenous antigens are taken up via endocytosis.

Antigens are degraded in endosomes/lysosomes.

MHC II is made in the ER and bound to an invariant chain (Ii) to prevent premature binding.

In endosomes, Ii is degraded, leaving CLIP in the binding groove.

HLA-DM helps remove CLIP and replace it with the actual peptide.

The MHC-peptide complex is moved to the cell surface.

Question 17

Q14: What is the main mechanism of immunity against bacteria?

Answer 17

Antibody-mediated immunity is central:

Neutralizes toxins

Promotes opsonization and phagocytosis

Low levels of bacteria may be cleared by phagocytes alone; high levels require adaptive response.

Question 18

Q15: What defenses are used against fungi?

Answer 18

Innate immunity: Phagocytosis and complement (lectin pathway)

Adaptive immunity: T cell and antibody involvement for persistent infections

Complement pathways enhance fungal clearance

Question 19

Q16: What are non-specific immune responses to viruses?

Answer 19

Interferons α and β: induce antiviral state

NK cells: kill infected cells

Macrophages/fibroblasts: secrete antiviral cytokines

Question 20

Q17: What are specific immune responses to viruses?

Answer 20

IgA: blocks mucosal entry

IgM, IgG, IgA: prevent viral fusion

IgG: enhances phagocytosis

IgM: agglutinates viral particles

IgG/IgM: activate classical complement pathway

IFN-γ: direct cytotoxic activity

CD8+ T cells: kill infected cells

Question 21

Q18: What is the lifecycle of malaria (Plasmodium)?

Answer 21

Sporozoites injected by mosquito enter the liver.

Develop into merozoites → infect RBCs → replicate → burst out.

Some develop into gametocytes → taken up by another mosquito

Question 22

Q19: What immune responses clear malaria?

Answer 22

Antibodies: Block sporozoite and merozoite invasion

IFN-γ and CD8+ T cells: Inhibit liver stage

NK cells: Kill parasitized RBCs and hepatocytes