diff --git a/prboom2/src/d_deh.c b/prboom2/src/d_deh.c
index c2226729e..4bdadf16f 100644
--- a/prboom2/src/d_deh.c
+++ b/prboom2/src/d_deh.c
@@ -1046,7 +1046,7 @@ typedef struct
 // killough 8/9/98: make DEH_BLOCKMAX self-adjusting
 #define DEH_BLOCKMAX (sizeof deh_blocks/sizeof*deh_blocks)  // size of array
 #define DEH_MAXKEYLEN 32 // as much of any key as we'll look at
-#define DEH_MOBJINFOMAX 25 // number of ints in the mobjinfo_t structure (!)
+#define DEH_MOBJINFOMAX 27 // number of mobjinfo configuration keys
 
 // Put all the block header values, and the function to be called when that
 // one is encountered, in this array:
@@ -1112,6 +1112,10 @@ static const char *deh_mobjinfo[DEH_MOBJINFOMAX] =
   "Bits2",               // .flags
   "Respawn frame",       // .raisestate
   "Dropped item",        // .droppeditem
+
+  // mbf21
+  "Infighting group",    // .infighting_group
+  "Projectile group",    // .projectile_group
 };
 
 // Strings that are used to indicate flags ("Bits" in mobjinfo)
@@ -1888,6 +1892,27 @@ static void setMobjInfoValue(int mobjInfoIndex, int keyIndex, uint_64_t value) {
       }
       break;
     case 24: mi->droppeditem = (int)(value-1); return; // make it base zero (deh is 1-based)
+
+    // mbf21
+    // custom groups count from the end of the vanilla list
+    // -> no concern for clashes
+    case 25:
+      mi->infighting_group = (int)(value);
+      if (mi->infighting_group < 0)
+      {
+        I_Error("Infighting groups must be >= 0 (check your dehacked)");
+        return;
+      }
+      mi->infighting_group = mi->infighting_group + IG_END;
+      return;
+    case 26:
+      mi->projectile_group = (int)(value);
+      if (mi->projectile_group < 0)
+        mi->projectile_group = PG_GROUPLESS;
+      else
+        mi->projectile_group = mi->projectile_group + PG_END;
+      return;
+
     default: return;
   }
 }
diff --git a/prboom2/src/dsda/global.c b/prboom2/src/dsda/global.c
index a75b66651..ac293d352 100644
--- a/prboom2/src/dsda/global.c
+++ b/prboom2/src/dsda/global.c
@@ -251,6 +251,10 @@ static void dsda_InitDoom(void) {
     mobjinfo[i].droppeditem  = mobjinfo_p->droppeditem;
     mobjinfo[i].crashstate   = 0; // not in doom
     mobjinfo[i].flags2       = 0; // not in doom
+
+    // mbf21
+    mobjinfo[i].infighting_group = IG_DEFAULT;
+    mobjinfo[i].projectile_group = PG_DEFAULT;
   }
 
   // don't want to reorganize info.c structure for a few tweaks...
@@ -264,6 +268,9 @@ static void dsda_InitDoom(void) {
   mobjinfo[MT_BABY].flags2    = MF2_MAP07BOSS2;
   mobjinfo[MT_BRUISER].flags2 = MF2_E1M8BOSS;
   mobjinfo[MT_UNDEAD].flags2  = MF2_LONGMELEE | MF2_RANGEHALF;
+
+  mobjinfo[MT_BRUISER].projectile_group = PG_BARON;
+  mobjinfo[MT_KNIGHT].projectile_group = PG_BARON;
 }
 
 static void dsda_InitHeretic(void) {
@@ -371,8 +378,16 @@ static void dsda_InitHeretic(void) {
     mobjinfo[j].droppeditem  = 0; // not in heretic
     mobjinfo[j].crashstate   = mobjinfo_p->crashstate;
     mobjinfo[j].flags2       = mobjinfo_p->flags2;
+
+    // mbf21
+    mobjinfo[j].infighting_group = IG_DEFAULT;
+    mobjinfo[j].projectile_group = PG_DEFAULT;
   }
 
+  // don't want to reorganize info.c structure for a few tweaks...
+  mobjinfo[HERETIC_MT_SORCERER2].infighting_group = IG_WIZARD;
+  mobjinfo[HERETIC_MT_WIZARD].infighting_group = IG_WIZARD;
+
   // heretic doesn't use "clip" concept
   for (i = 0; i < NUMAMMO; ++i) clipammo[i] = 1;
 
diff --git a/prboom2/src/info.h b/prboom2/src/info.h
index 95f213ac0..cacc2f7f5 100644
--- a/prboom2/src/info.h
+++ b/prboom2/src/info.h
@@ -2995,6 +2995,19 @@ typedef enum {
   TOTAL_NUMMOBJTYPES = HERETIC_NUMMOBJTYPES
 } mobjtype_t;
 
+typedef enum {
+  IG_DEFAULT,
+  IG_WIZARD,
+  IG_END
+} infighting_group_t;
+
+typedef enum {
+  PG_GROUPLESS = -1,
+  PG_DEFAULT,
+  PG_BARON,
+  PG_END
+} projectile_group_t;
+
 /********************************************************************
  * Definition of the Thing structure
  ********************************************************************/
@@ -3055,6 +3068,10 @@ typedef struct
   // heretic
   int crashstate;
   uint_64_t flags2;
+
+  // mbf21
+  int infighting_group;
+  int projectile_group;
 } mobjinfo_t;
 
 typedef struct
diff --git a/prboom2/src/mbf21.md b/prboom2/src/mbf21.md
index 57304cd37..d2c3646b7 100644
--- a/prboom2/src/mbf21.md
+++ b/prboom2/src/mbf21.md
@@ -52,6 +52,44 @@ This is proof-of-concept implemented in dsda-doom.
 #### Option default changes
 - comp_pursuit: 1 (was 0)
 
+#### Dehacked Thing Groups
+
+##### Infighting
+- Add `Infighting group = N` in Thing definition.
+- `N` is a nonnegative integer.
+- Things with the same value of `N` will not target each other after taking damage.
+
+##### Projectile
+- Add `Projectile group = M` in Thing definition.
+- `M` is an integer.
+- Things with the same value of `M` will not deal projectile damage to each other.
+- A negative value of `M` means that species has no projectile immunity, even to other things in the same species.
+
+##### Examples
+
+```
+Thing 12 (Imp)
+Projectile group = -1
+
+Thing 16 (Baron of Hell)
+Projectile group = 2
+
+Thing 18 (Hell Knight)
+Infighting group = 1
+Projectile group = 1
+
+Thing 21 (Arachnotron)
+Infighting group = 1
+Projectile group = 2
+```
+
+In this example:
+- Imp projectiles now damage other Imps (and they will infight with their own kind).
+- Barons and Arachnotrons are in the same projectile group: their projectiles will no longer damage each other.
+- Barons and Hell Knights are not in the same projectile group: their projectiles will now damage each other, leading to infighting.
+- Hell Knights and Arachnotrons are in the same infighting group: they will not infight with each other, despite taking damage from each other's projectiles.
+- Note that the group numbers for infighting and projectiles are separate - being in infighting group 1 doesn't mean you are in projectile group 1.
+
 #### New Thing Flags
 
 Implementations match between DSDA-Doom and Eternity Engine,
diff --git a/prboom2/src/p_inter.c b/prboom2/src/p_inter.c
index 830d42d2f..b94c33d7b 100644
--- a/prboom2/src/p_inter.c
+++ b/prboom2/src/p_inter.c
@@ -891,6 +891,13 @@ static void P_KillMobj(mobj_t *source, mobj_t *target)
 // and other environmental stuff.
 //
 
+static dboolean P_InfightingImmune(mobj_t *target, mobj_t *source)
+{
+  return // not default behaviour, and same group
+    mobjinfo[target->type].infighting_group != IG_DEFAULT &&
+    mobjinfo[target->type].infighting_group == mobjinfo[source->type].infighting_group;
+}
+
 void P_DamageMobj(mobj_t *target,mobj_t *inflictor, mobj_t *source, int damage)
 {
   player_t *player;
@@ -1177,7 +1184,7 @@ void P_DamageMobj(mobj_t *target,mobj_t *inflictor, mobj_t *source, int damage)
     (!target->threshold || target->flags2 & MF2_NOTHRESHOLD) &&
     ((source->flags ^ target->flags) & MF_FRIEND || monster_infighting || !mbf_features) &&
     !(heretic && source->flags2 & MF2_BOSS) &&
-    !(target->type == HERETIC_MT_SORCERER2 && source->type == HERETIC_MT_WIZARD)
+    !P_InfightingImmune(target, source)
   )
   {
     /* if not intent on another player, chase after this one
diff --git a/prboom2/src/p_map.c b/prboom2/src/p_map.c
index 9ab90e6eb..194b41e44 100644
--- a/prboom2/src/p_map.c
+++ b/prboom2/src/p_map.c
@@ -510,6 +510,25 @@ dboolean PIT_CheckLine (line_t* ld)
 // PIT_CheckThing
 //
 
+static dboolean P_ProjectileImmune(mobj_t *target, mobj_t *source)
+{
+  return
+    ( // PG_GROUPLESS means no immunity, even to own species
+      mobjinfo[target->type].projectile_group != PG_GROUPLESS ||
+      target == source
+    ) &&
+    (
+      ( // target type has default behaviour, and things are the same type
+        mobjinfo[target->type].projectile_group == PG_DEFAULT &&
+        source->type == target->type
+      ) ||
+      ( // target type has special behaviour, and things have the same group
+        mobjinfo[target->type].projectile_group != PG_DEFAULT &&
+        mobjinfo[target->type].projectile_group == mobjinfo[source->type].projectile_group
+      )
+    );
+}
+
 static dboolean PIT_CheckThing(mobj_t *thing) // killough 3/26/98: make static
 {
   fixed_t blockdist;
@@ -624,17 +643,10 @@ static dboolean PIT_CheckThing(mobj_t *thing) // killough 3/26/98: make static
     if (tmthing->z + tmthing->height < thing->z)
       return true;    // underneath
 
-    if (
-      tmthing->target &&
-      (
-        tmthing->target->type == thing->type ||
-        (tmthing->target->type == MT_KNIGHT && thing->type == MT_BRUISER) ||
-        (tmthing->target->type == MT_BRUISER && thing->type == MT_KNIGHT)
-      )
-    )
+    if (tmthing->target && P_ProjectileImmune(thing, tmthing->target))
     {
       if (thing == tmthing->target)
-        return true;                // Don't hit same species as originator.
+        return true;                // Don't hit self.
       else
         // e6y: Dehacked support - monsters infight
         if (thing->type != g_mt_player && !monsters_infight) // Explode, but do no damage.