Based on presence/absence of antigens (chemicals that induce immune response) on RBC surface and antibodies (proteins produced in response to antigens) in plasma.

ABO Grouping:

Blood Group	Antigens on RBCs	Antibodies in Plasma	Can Donate To	Can Receive From
A	A	anti-B	A, AB	A, O
B	B	anti-A	B, AB	B, O
AB	A, B	None	AB	A, B, AB, O (Universal Recipient)
O	None	anti-A, anti-B	A, B, AB, O (Universal Donor)	O

Rh Grouping:
- Rh antigen (D antigen): Present in Rh-positive ($Rh^+$) individuals (approx. 80%). Absent in Rh-negative ($Rh^-$) individuals.
- Anti-Rh antibodies are NOT naturally present in $Rh^-$ individuals. They are produced only upon exposure to $Rh^+$ blood.
- Erythroblastosis foetalis: Severe hemolytic disease of the newborn.
  - Occurs when $Rh^-$ mother carries $Rh^+$ foetus.
  - During first pregnancy, mother's blood usually doesn't mix significantly with foetal blood, so she may develop antibodies only after delivery.
  - In subsequent $Rh^+$ pregnancies, maternal anti-Rh antibodies cross placenta, destroy foetal RBCs, leading to severe anemia and jaundice.
  - Prevention: Administer anti-Rh antibodies to mother immediately after delivery of first $Rh^+$ baby to destroy any foetal RBCs in her circulation.

Mechanism to prevent excessive blood loss.
Process:
1. Injury to blood vessel triggers release of clotting factors (thromboplastin) from damaged tissues and platelets.
2. Clotting factors, in presence of $Ca^{2+}$, activate prothrombin activator complex.
3. Prothrombin activator converts inactive prothrombin (plasma protein) to active thrombin.
4. Thrombin converts soluble fibrinogen (plasma protein) into insoluble fibrin monomers.
5. Fibrin monomers polymerize to form a network of threads.
6. Fibrin threads trap RBCs and platelets to form the clot (thrombus).
Key components: Platelets, Calcium ions ($Ca^{2+}$), Vitamin K (for synthesis of clotting factors).

Formation: As blood flows through capillaries, water, small soluble substances (nutrients, gases, hormones) filter out of capillaries into interstitial spaces, forming tissue fluid (interstitial fluid).
Composition: Similar to plasma but lacks large proteins and RBCs. Contains WBCs (lymphocytes).
Function:
- Medium for exchange between blood and cells.
- Drains into lymphatic capillaries, forming lymph.
- Carries absorbed fat from intestine (lacteals).
- Transports lymphocytes and antibodies for im