Question

1. What signals activate cytotoxic T cells?
2. Describe different types of vaccines?
3. Discuss the differences between the alternative and classical pathways of complement activation.

Answer 1

ANSWER

1) Signals that activate cytotoxic T cells:

   T cells are generated in the Thymus, and they are specefic for one particular foreign particle (antigen). When they reach circulation, they recognise their antigen on the surface of Antigen Presenting Cells (APC). The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen on MCH (Major Histocompatibility Complex) on the surface of the APC.

• The T cell receptor on a CD8+ T cell recognizes the pathogen peptide displayed on the surface of an infected cell.
• The T cell releases cytotoxic molecules that kill the infected cell and stop the pathogen from spreading.

SIGNAL	T Cell	APC (Antigen Presenting Cell)
First signal	TCR (T cell receptor)	peptide-bound MHC class I molecule
Second signal	CD28 molecule on the T cell	Either CD80 or CD86

2) Different types of vaccines are:

   (i) Live, attenuated vaccines:- Contains a version of the living microbe, that has been weakend in the lab, so that it can no longer cause disease, but will provoke an immune response that can protect against future infection. Examples:- Vaccines against Measles, Mumps and Rubella (MMR), Varicella, etc.

   (ii) Inactivated/Killed vaccines:- They are produced by inactivating or killing the disease-causing microbe with chemicals, heat, or radiation. These are more stable and safer than live vaccines because, this destroys the pathogen’s ability to replicate, but keeps it “intact” so that the immune system can still recognize it. Killed vaccines tend to provide a shorter length of protection than live vaccines, and are more likely to require boosters to create long-term immunity. Examples:- Polio (IPV), Hepatitis A, Rabies, etc.

   (iii) Subunit vaccines:- Here, instead of entire microbe, it includes only the antigens that best stimulate the immune system. Examples:- The acellular pertussis vaccine, influenza vaccine, Plague immunization

   (iv) Toxoid vaccines:- These vaccines are used for certain bacterial diseases which are not directly caused by a bacterium itself, but are caused by the toxin produced by the bacterium. Examples:- Diphtheria, Tetanus immunization.

   (v) Conjugate vaccines:- They are produced from pieces from the coats of bacteria (That is an outer coating of sugar molecules called polysaccharide). Polysaccharide coatings disguise a bacterium's antigens so that the immature immune systems of infants and younger children can't recognize or respond them to them. Example:- Haemophilus influenza type B vaccine.

3) Differences between the alternative and classical pathways of complement activation:

Feature	Classical pathway	Alternative pathway
Type of immunity	Acquired (Specefic)	Innate (Non-specefic)
Initiation	Antigen-Antibody complex	Microbial components (e.g. Endotoxin)
Role of antibodies	Needed for initiation (Activation of C1)	No role
Role of Properdin	No role	Needed for activation of C3
C3 convertase	C4b2b	C3bBb
C5 convertase	C4b2b3b	C3bBb3b
Involved components	C1 - C9	Factor B, Factor D, Properdin, C3, 5-9
MAC (C5b6789)	Formed	Formed